Random-subspace tests and trace smoothing for coordinate sweeps
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A self-contained introduction, with the full proof, aimed at a reader
who has seen a little linear algebra, group actions, and probability.
0. A note to the reader
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This is a proof that a certain shuffling procedure is fast. The proof
uses some genuinely non-elementary tools (Schur-Weyl duality, Schatten
norms, hook-length formulas), so first-time readers should read
sections 1-3 for the idea, then read section 4 onward as the detailed
argument. Every lemma and proposition is fully stated (with its
hypotheses) where it is first used, and every proof is complete, with
a few external inputs clearly labeled as "Input" from a companion
paper.
Notation is standard. Comments in brackets like [why?] explain a step
that a careful reader might want to check. Short "in plain words"
boxes (marked >>) restate a technical step in everyday language.
1. What is the Thorp shuffle?
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Take n = 2^d cards, and lay them on the 2^d corners of a d-dimensional
cube. Each corner is a binary string of length d. One physical
shuffle does three things:
(i) for every corner v that ends in 0, flip a fair coin xi(v) and
use it to decide a "swap": the cards at v and at v+xi(v)*e_d
(the partner across the last coordinate) either trade
places or stay put -- one specific behavior per coin.
(ii) rotate every corner left by one binary coordinate: (x_1,...,x_d)
becomes (x_2,...,x_d,x_1).
(iii) done. Repeat d such physical shuffles and the rotation has
come back to where it started.
Equivalently: a "coordinate sweep" applies, for every coordinate
i = 1,2,...,d, the random coordinate matching between the two halves
of the cube cut out by the i-th coordinate. The coordinates are done
in sequence, and the d physical shuffles one after another make one
sweep. Then you start over.
We write Q_n for the average over all coin flips of one full sweep: a
probability distribution on the symmetric group S_n, viewed as an
operator on functions of the deck (or on any representation of S_n).
Why would anyone care? Morris and others proved that O(log^4 n)
sweeps suffice (log^4 n ~ d^4 for n = 2^d) using entropy; the present
paper shows O(d) sweeps suffice and that this order is optimal. Each
sweep is d physical shuffles, so O(d) sweeps = O(d^2) physical
shuffles; that is the famous folklore conjecture for Thorp mixing, up
to order in the exponent of d.
2. The main theorem, and how to read it
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We measure mixing by the total variation distance: for two
probability measures mu, nu on S_n,
||mu - nu||_TV = (1/2) sum_{g in S_n} |mu(g) - nu(g)|,
which takes values in [0,1], is 1 for disjoint supports, and is 0
exactly when mu = nu.
For a permutation-average Q_n we define its action Q_n(lambda) on each
irreducible representation V_lambda of S_n by averaging over
permutations in the support of Q_n. For a matrix Y we set
|Y| = (Y^* Y)^{1/2}, and for p >= 1 we define the Schatten p-norm
||Y||_p = (Tr |Y|^p)^{1/p}; all traces in this paper are
unnormalized.
The regular trace of a group-algebra element X on S_n is
Tr_reg(X) = sum_{lambda partition of n} D_lambda * Tr_{V_lambda} (X(lambda)),
where D_lambda = dim V_lambda. It is literally the trace of X acting
on the regular representation, in which each V_lambda appears D_lambda
times. The trivial representation (all of S_n acting trivially)
contributes exactly 1 to Tr_reg|Q_n|^P: Q_n acts as the identity
there, and |1|^P = 1.
Theorem A (regular-trace smoothing).
There is an absolute constant P >= 2 such that for every dyadic
n = 2^d >= 2,
Tr_reg|Q_n|^P <= 1 + n^{-10}. (2.1)
Moreover, for any integer v with 2v >= P, the law mu_v of v
independent sweeps satisfies
||mu_v - U_{S_n}||_TV <= (1/2) n^{-5},
where U_{S_n} is the uniform distribution on S_n. The bound is
uniform over every deterministic initial deck.
In plain words: after an absolute (i.e. n-independent) number of
sweeps, the regular-trace moment sits within n^{-10} of what it would
be for the uniform distribution, and this converts to a total-variation
distance that decays as n^{-5}.
Corollary B (order-optimal mixing).
One sweep is d physical shuffles, so Theorem A gives O(d) physical
shuffles sufficient. We show in section 8 that no algorithm can
mix in fewer than 2d - O(1) physical shuffles; so O(d) is optimal
in order of d.
3. Roadmap of the proof
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The proof controls the trace in (2.1) by splitting the sum over
partitions into three groups. Writing X_lambda = log D_lambda,
(A) LARGE DIMENSION: X_lambda >= A_1 * n.
Handled by the coarse-smoothing estimate (Proposition 4.1),
which bounds the moment of a fixed power by e^{Cn} / D_lambda.
(B) VERY LONG FIRST ROW: 1 <= n - lambda_1 <= n * e^{-M*sqrt(log n)}.
Handled by a sparse-row estimate (Proposition 5.1): the
symmetric group's permutation module on a small partial deck
contains lambda with high multiplicity, and the moment there
is bounded by D_lambda^{\-A} for any fixed A.
(C) THE REST: ne^{-A_0*sqrt(log n)} <= X_lambda <= A_1 * n.
Handled by the grid-overlap estimate (Proposition 6.1). We
split the coordinates into balanced groups to see a sweep as
a composition of row and column shuffles, and use random
low-dimensional subspaces to control the overlap of the two
symmetry actions.
The three pieces are stitched together using a version of the
Araki-Lieb-Thirring inequality (Proposition 7.2): for positive
matrices B, C and exponent r >= 1,
Tr |B C|^r <= Tr (B^* B)^{r/2} (C C^*)^{r/2}.
The left side is bounded via row and column Schatten moments
(one Schro\"dinger for each side) using the corresponding subgroup
estimates; the "overlap" between the two sides is exactly what the
grid test measures.
Finally, the induction runs on the bit length d = log_2 n. Splitting
d-bit strings along the middle coordinate gives a balanced (a,b)
grid with a,b both about sqrt(n), and the induction hypothesis applies
at bit lengths floor(d/2) and ceil(d/2). The exponent P(n) grows by
a factor (1 + 2 (log n)^{-1/4}) at each step, but the product over
halvings is uniformly bounded, giving an absolute P in Theorem A.
The lower bound (section 8) is elementary: one physical shuffle
uses tn/2 independent coin bits, so the support of t physical
shuffles contains at most 2^{tn/2} permutations; no distribution
supported on such a small set can be close to uniform when t < 2d
is too small.
4. Input: partial-deck smoothing
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The first estimate is imported from a companion paper and is the only
external input in the whole proof.
Input 4.1 (Corollary 6.2 of "Routing densities and representation
contraction for Thorp sweeps").
Let n = 2^d, d >= 0, and let Q_n be a sweep in any fixed order of
the d coordinates. For either B_n = Q_n^* Q_n or B_n = Q_n Q_n^*,
there exist absolute constants u >= 1 (an integer) and C < infinity
such that for every pair of ordered injections x, y of l distinct
cards, 0 <= l <= n,
B_n^u(x,y) <= e^{C l} / (n)_l. (4.1)
Here (n)_l = n!/(n-l)! is the falling factorial, with (n)_0 = 1.
The same constants (u, C) work for every coordinate order.
Consequence: taking l = n in (4.1) gives
Tr_reg |Q_n|^{2u} <= e^{C n}. (4.2)
Proof of the consequence. When l = n, the "injection module of
size n" is literally the regular representation of S_n. There are
n! ordered injections of length n (that is, n! distinct permutations
of the n cards) and each diagonal entry of B_n^u there is at most
e^{C n}/n!. Summing the n! entries gives at most e^{C n}. Finally
(Q_n^*Q_n)^u = (Q_n^*Q_n)^{u} = |Q_n|^{2u} (Schatten powers). []
Interpretation. B_n^u is very close to (a power of) the uniform
distribution on the partial injection: it is at most e^{C l} times
the uniform probability 1/(n)_l at every point. The "classical
Thorp shuffle applied to l cards on a full deck of n positions" is
what this measures.
>> In plain words. Take a sweep, take any power of it, take the
>> squared singular values in Schatten form. The result is an operator
>> that is uniformly very small (roughly uniform on its part of the
>> deck, up to an exponential correction e^{C l}.
This single estimate is what makes the "large dimension" and
"moderate but still non-negligible dimension" cases work.
5. Long first rows: a sparse-row estimate
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Now consider partitions lambda of n whose first row is almost the
whole diagram: n - lambda_1 = k is small relative to n. Such
partitions have extremely high dimension but appear with very high
multiplicity in the permutation module on a partial deck. Because
that module is small (its base is a small subset of S_n), controlling
its Schatten moments gives a big win once the multiplicity is
correctly tracked.
The precise statement is Proposition 5.1:
Proposition 5.1 (Sparse-row contribution).
There exist absolute constants M > 0 and P_s >= 2 such that, for
all sufficiently large dyadic n,
sum over {lambda partition of n : 1 <= n - lambda_1 <= n e^{-M sqrt(log n)}}
D_lambda * Tr_{V_lambda} |Q_n(lambda)|^{P_s} <= n^{-12}. (5.1)
The rest of this section proves Proposition 5.1.
5.1 A deformation on partial decks
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Fix p_0 >= 4u with p_0 > 2, where u is from Proposition 4.1.
We work on the space V_q of ordered q-tuples of distinct cards
("ordered injections" of length q). The action of S_n is the natural
one on the underlying set of positions; the space V_q carries the
induced (permutation) representation on the q-tuple.
Partition the d coordinates into consecutive blocks of length about
sqrt(d). There are g = O(sqrt(log n)) such blocks. Each block
corresponds to a set of "coarse" positions of size b_i = 2^(block size),
and b_i is at most exp(C sqrt(log n)) for a universal C.
Definition (coarse update). "Updating coarse coordinate i" means:
for every line parallel to block i (i.e., for every choice of all
fine coordinates outside block i), apply an independent cube sweep
Q_{b_i} on that line. The lines are joint by sweep order, but
since a single block covers exactly one coarse coordinate, each
line's content is updated independently. This is one factor in the
full sweep decomposition.
Now consider the extension to complex z, 0 <= Re z <= 1. Write each
line operator Y = W |Y| in its polar decomposition. Define
Y(z) = W |Y|^{p_0 z / 2}, 0 <= Re z <= 1,
with the value zero on the nullspace of Y. Let Q(z) be the product
of these operators in sweep order.
Boundary values:
- At z = 2/p_0, Y(z) = W |Y| = Y (the original placement sweep).
- On Re z = 0, every factor has operator norm <= 1, and so does
the product.
- On Re z = 1, within one l-card line-block, singular-value
Cauchy-Schwarz gives
|Y(z)_{yx}| <= ((|Y|^{p_0/2})_{xx} (|Y^*|^{p_0/2})_{yy})^{1/2}
<= e^{C l} / (b_i)_l. (5.2)
Here the second inequality uses Proposition 4.1 twice: once with
B_i = Y^* Y (so |Y|^{p_0/2} is bounded by |Y|^{2u}, because
p_0/2 >= 2u), once with B_i = Y Y^* (for |Y^*|^{p_0/2}). For l = 0
and l = 1 the exponential factor e^{C l} is unnecessary (it is 1):
a single card is exactly uniform after the full line has been
swept, since the sweep applies every coordinate matching at least
once, and a single-card distribution is uniform after one sweep.
The deformation leaves this exactness untouched because W|Y|^0 = W
has exactly the same effect on one-card inputs as W|Y| does.
5.2 Contact counts for the deformed kernel
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Now we compare Q(z) with an auxiliary kernel K(x, y) which uses an
independent uniform permutation on every coarse line:
K(x, y) = product over coarse lines of 1 / (b_i)_{l_i},
where l_i = number of distinguished cards in line i on both ends, and
K(x,y) = 0 if an intermediate placement has a collision.
Let U(x, y) = sum over coarse lines of l_i * 1_{l_i >= 2}; it counts
"visits of a coarse line by at least two distinguished cards". Use
U(x,y) = 0 for invalid paths.
Claim (5.3). On Re z = 1,
|Q(z)_{yx}| <= K(x,y) * e^{C U(x,y)}, (5.3)
K(x,y) <= n^{-q} * e^{C U(x,y)}. (5.3b)
Proof of (5.3): each line contributes at most e^{C l_i}/(b_i)_{l_i}
by (5.2), and the product over lines gives the extra factor
e^{C sum l_i} = e^{C U} once we use l_i = 1 for the boundary terms.
Proof of (5.3b): compare (b_i)_{l_i} with b_i^{l_i}:
b_i^{l_i} / (b_i)_{l_i} = (1 - 1/b_i)(1 - 2/b_i)...
<= (l_i)^{l_i} / l_i! <= e^{l_i}, with equality when l_i <= 1.
Multiply over lines: the product of b_i^{-l_i} is n^{-q} when all
the q distinguished cards land on distinct lines, and the product
of the 1/(b_i)_{l_i} is at most n^{-q} times e^{U}. []
Combined bound. Square (5.3), then use (5.3b) on one factor of
K:
|Q(z)_{yx}|^2 <= K(x,y) * e^{C U(x,y)} * |Q(z)_{yx}|
<= n^{-q} * e^{C U(x,y)} * |Q(z)_{yx}|
Summing over y, and writing the sum as an expectation over a
uniformly random injection x and independent random line
permutations producing the y-values,
||Q(z)||_{HS}^2 <= (n)_q * n^{-q} * E[ e^{C U} ]. (5.4)
Here E is over the uniform input injection x and the independent
uniform line permutations defining K.
5.3 The contact-count bound
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Condition on all line permutations, and trace the paths of all n
inputs through the sweep. Join two inputs if they ever share a line
in some coarse block. Result: a graph G whose maximum degree is at
most
Delta = sum_i (b_i - 1) <= exp(C'' sqrt(log n)). (5.5)
The distinguished q inputs form a uniformly random q-subset of the
n vertices of G. Let R_* = number of selected vertices that have at
least one selected neighbor. Then U <= g R_*, because each visit
counted by U is at most one block wide and a vertex visited at least
once in a block where it shares a line counts once in R_*.
Bound E[e^{C g R_*}]. For a set S of size w, the number of connected
subsets of G of size w is at most n * Delta^{2(w-1)} (encode S by
a rooted spanning-tree traversal). Any fixed union of v <= q
vertices is entirely selected with probability
(q)_v / (n)_v <= (q/n)^v,
and with probability zero if v > q.
The nontrivial components of the selected graph are a disjoint family
of connected sets, each of size at least 2. Therefore e^{C g R_*} is
dominated by
sum over families {S_1, S_2, ..., S_m} of disjoint connected sets
of size >= 2, whose union is contained in the selected set,
of the product product_j e^{C g |S_j|}.
Drop disjointness and factorize as an exponential sum:
E[e^{C g R_*}]
<= exp( sum_{w >= 2} n * Delta^{2(w-1)} * (q/n)^w * e^{C g w} ).
Since Delta^{2} <= e^{2 C'' sqrt(log n)} and g = O(sqrt(log n)), for
a sufficiently large absolute C_2 the terms are bounded by
(q/n) * e^{C_2 sqrt(log n)} <= 1/2.
Therefore
E[e^{C g R_*}] <= exp( n * (q^2/n) * e^{C_3 sqrt(log n)} )
= exp( (q^2/n) * e^{C_3 sqrt(log n)} ). (5.6)
[Here I used: sum over w >= 2 of (const)^w is (const)^2 times a
bounded factor, and Delta^{2(w-1)} * e^{C g w} <= e^{C_2 sqrt(log n) * w}.]
Combining (5.4) with (5.6), we get a Hilbert-Schmidt bound on
Re z = 1. Interpolate the operator-norm bound on Re z = 0 with this
boundary. At z = 2/p_0, the interpolation gives a Hilbert-Schmidt
norm bound, so the Schatten exponent is p_0:
Tr_{q placements} |Q_n|^{p_0}
<= exp( (q^2/n) e^{C_3 sqrt(log n)} ). (5.7)
The three-lines argument (Stein's): pair Q(z) with a dual Schatten
unit matrix of exponent p_0' = p_0/(p_0-1), raise its singular values
to p_0'(1 - z/2), leave zeros zero. The pairing has trace norm one
at Re z = 0, Hilbert-Schmidt norm one at Re z = 1, and recovers the
original dual matrix at z = 2/p_0. Raise the interpolated norm bound
to p_0, and you get (5.7).
>> The trick: we know Q is Hilbert-Schmidt on Re z = 1 with an
>> exponential bound depending only on the "contacts" between paths,
>> and we know Q is a contraction on Re z = 0. Interpolation gives
>> a Schatten p_0 bound at z = 2/p_0.
5.4 From placement moments to regular multiplicities
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Proof of Proposition 5.1.
For k = n - lambda_1 in the sparse range, put
q = floor( k * (n/k)^{1/5} ). (5.8)
For M large enough in the hypothesis of (5.7), the bound is
applicable and its right side is at most e^k: indeed q^2/n <=
k (k/n)^{3/5}, and (k/n)^{3/5} <- log(n/k) when log(n/k) grows.
Also q/k -> infinity and q/n -> 0 uniformly over the sparse range.
Let beta = (lambda_2, lambda_3, ...) be a partition of k. The
stabilizer of an ordered q-card placement is S_{n-q}, so branching of
the action on V_q identifies the multiplicity m_lambda of V_lambda
with the number of standard tableaux of the skew shape lambda/(n-q).
For large n, the leftover first-row segment is well separated from
beta (indeed n - q >= beta_1), so by the branching rule
m_lambda = binom(q, k) * D_beta. (5.9)
The hook-length formula, with beta' the transpose of beta, gives
D_lambda = binom(n, k) * D_beta *
product over i = 1, ..., beta_1 of
( 1 + beta'_i / (n - k - i + 1) )^{-1}. (5.10)
Since sum_i beta'_i = k and k = o(n), the product of logarithms
loses at most k/(n - 2k + 1), so in particular
log D_lambda >= (1/2) k log(n/k) (5.11)
for all sufficiently large n in the sparse range.
The multiplicity is a substantial power of the dimension:
log m_lambda >= (1/5) k log(n/k) - O(k) + log D_beta,
log D_lambda <= k log(n/k) + k + log D_beta.
Because log(n/k) -> infinity uniformly (i.e., over the whole sparse
range, not just a subsequence), these give
m_lambda >= D_lambda^{1/10}. (5.12)
Now sum over ordered q-placements. The trace on V_q includes m_lambda
copies of the V_lambda block. Using (5.12) and (5.7):
Tr_{V_lambda} |Q_n(lambda)|^{p_0}
<= e^k * D_lambda^{-1/10}
<= D_lambda^{-1/20}. (5.13)
For any fixed large A, choose a fixed P_s with P_s / p_0 >= 20(A+1).
The power-sum inequality sum_j a_j^t <= (sum_j a_j)^t (for
a_j >= 0 and t >= 1) then gives
D_lambda * Tr_{V_lambda} |Q_n(lambda)|^{P_s}
<= D_lambda^{-A}. (5.14)
To finish. There are at most 2^k possible tails beta of size k.
For k <= sqrt(n), the dimension lower bound (5.11) makes their
total contribution at most
sum_{k >= 1} (2 n^{-A/4})^k.
For k > sqrt(n), use log(n/k) >= M sqrt(log n); then the
contribution is at most
sum_{k > sqrt n} exp( k log 2 - (1/2) A M k sqrt(log n) ).
Take A sufficiently large; the sum of these two bounds is at most
n^{-12} for all sufficiently large n. This proves Proposition 5.1. []
6. The grid-overlap estimate
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This is the heart of the proof. We split the coordinates into an
(a, b) board and show that row and column symmetry actions have almost
disjoint ranges of singular values inside any irreducible
representation, up to a controlled exponential factor.
Setup 6.0.
Split the n positions into an a-by-b grid, with ab = n and
sqrt(n)/2 <= a, b <= 2 sqrt(n). (6.0)
Write R = (S_b)^a for the subgroup acting on the rows (each row is a
copy of S_b) and F = (S_a)^b for the subgroup acting on the columns.
These commute? No -- they do not commute as subgroups of S_n; they
are two decompositions of the same symmetric group into different
products of Young subgroups. So we must handle their actions
carefully.
We use Fourier projections on R and on F. Let V_mu be a real
orthogonal irreducible representation of R, let v_R in V_mu be a
real unit vector, and let D_R = dim V_mu. In the restriction of a
fixed irreducible V_lambda of S_n to R, let E_R act as
|v_R> 0. For all sufficiently large n, if
n e^{-A_0 sqrt(log n)} <= X := log D_lambda <= A_1 n, (6.1)
then for every pair of projections (E_R, E_F) just defined,
D_lambda * Tr_{V_lambda}(E_R E_F)
<= D_R D_F * exp( X / (log n)^{1/3} ). (6.2)
The rest of this section proves Proposition 6.1.
Overview of the proof.
The strategy is to view Tr(E_R E_F) as the square-norm of a single
invariant tensor phi, up to elementary factors. Then we bound that
square-norm in two ways. On the "retention" side, a random
low-dimensional test preserves a controlled fraction of the
lambda-component. On the "compression" side, the tensor is forced
to lie in a small subspace after the test, and a Fourier-analytic
compression costs exp(-n). The bookkeeping is:
- Normalization (Stirling). Converting overlap to tensor norm
costs (n!) / (|R| |F|) <= exp(n + O(sqrt(n) log n)).
- Retention. The random planes must cumulatively retain at
least a fraction alpha_H alpha_H' of the lambda-component, where
log(1/(alpha_H alpha_H')) is only O(n^{0.6}).
- Compression. The cell-tag structure saves exp(-n) (with the
dimension window supplying the remaining slack).
The +n from the first line is canceled by the -n from the third.
The remaining slack X / (log n)^{1/3} is compared with the true
window (6.1).
6.1 The tagged realization
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Put r = floor(n^{1/100}). For a partition nu, define s(nu) as the
number of boxes of nu strictly below its first r rows and strictly to
the right of its first r columns. In other words, s(nu) is the
number of boxes of nu that lie outside the r-by-r square anchored at
the top-left corner.
Claim 6.1a. With s = s(lambda), we have (for sufficiently large n)
s <= C X / log n (6.3)
for an absolute C.
Proof. Tile the diagram by r-by-r squares, aligned at the top-left.
Each tile that meets the counted boxes has, immediately up and to
the left, a completely filled tile; so the counted boxes are
supported by at least s full tiles. Each full r-by-r tile
contributes at least r^2 (log r - O(1)) to the logarithm of the
tableau count D_lambda (standard hook-length estimate). Ordering
the tiles by row and then by column, and filling partial tiles in
any fixed legal way, respects the Young order on tableaux. So
D_lambda >= exp( r^2 (log r - O(1)) * (number of full tiles) )
>= exp( r^2 (log r - O(1)) * s / r^2 )
= exp( (log r - O(1)) * s ).
Since log D_lambda = X and r = n^{1/100}, we get
log r = (1/100) log n, and so
s <= X / ((1/100) log n - O(1)) <= (200) X / log n for large n.
[]
The rest of this subsection builds a tensor realization of V_lambda
which lets us compute Tr(E_R E_F).
Definition (graded alphabet action). Hilbert tensor powers of an
alphabet with even and odd letter spaces carry a graded symmetric
group action. For homogeneous letters u, v set
u (x) v |-> (-1)^{p(u) p(v)} v (x) u
on adjacent slots, (6.4)
where p in {0, 1} is the parity of a letter in its good/exceptional
class. This action extends to all of S_n by transpositions; it is
unitary; whenever nonconsecutive slots are grouped together, we use
the same graded reordering. In particular it carries actions within
those slot sets to tensor-product actions, and carries parity-stable
tag subspaces to the corresponding reordered tag subspaces.
Definition (r-hook alphabet). Call an alphabet with r evenly graded
"good" letters of one parity, r evenly graded "good" letters of the
other parity, and any number of "exceptional" even letters, an
r-hook alphabet. The r-hook alphabet realizes precisely the shapes
nu contained in the r-hook (first r rows union first r columns): this
is the classical Berele-Regev statement, restated in Regev 2013,
Theorem 2.1. If, in addition to the good letters, we have at least
s(nu) exceptional even letters, then nu can be realized using
exactly s(nu) exceptional slots, and no fewer. []
Sketch. By Pieri, the good even letters first supply at most r
rows. The good odd letters add at most r vertical strips, so the
r-hook is filled. Each additional exceptional slot adds at most
one box outside the resulting hook. Conversely, build the top r
rows by the even letters, then the remaining hook columns by the
odd letters, then the outside boxes with distinct exceptional
letters. []
Realize lambda with exactly s = s(lambda) exceptional slots. Restrict
to R. If the chosen R-type mu has row components mu_1, ..., mu_a,
it occurs in some sector of lambda whose exceptional slots are split
among the rows. Counting the minimum exceptional load of each row
gives
h_A := sum_i s(mu_i) <= s. (6.5)
The same argument applies to the column components.
Now for each i choose an alphabet A_i with r good letters of each
parity and s(mu_i) exceptional even letters. Set A = oplus_i A_i.
The good even space of A has dimension H = a r (r good letters per
row, a rows). The good odd space of A likewise has dimension H.
Embed the row representation (V_{mu_i}^{tensor multiplicity}) inside
the tensor power A_i^{otimes b}. The tensor product of these
embeddings realizes the whole chosen R-representation on A^{otimes n},
and hence realizes the specific unit carrier vector v_R (which might
be entangled across rows). Denote the image by psi in A^{otimes n}.
Then psi is a unit vector with exactly h_A exceptional slots, and the
slot in row i has tag A_i.
For g in R, set f_R(g) = , where g acts in the chosen row
representation. Because a permutation not preserving the row
structure changes the tag of at least one slot, the matrix coefficient
of psi vanishes outside R:
= f_R(g) for g in R, 0 otherwise. (6.6)
Analogously, we build a column-tagged unit vector eta in B^{otimes n},
where B = oplus_j B_j, with alphabet B_j of r good letters of each
parity and s(nu_j) exceptional even letters, and hence
h_B = sum_j s(nu_j) <= s. Its coefficient on F is
f_F(g) = , and it vanishes outside F.
6.2 A normalization: overlap as a tensor norm
----------------------------------------------------------------
On A^{otimes n} (x) B^{otimes n}, let P_* be the projection onto
"diagonal invariants": vectors invariant under the diagonal action
of S_n acting on both tensor factors simultaneously. Let
P_A^lambda, P_B^lambda be the isotypic projections onto lambda on the
A and B sides. Put
e = psi (x) eta,
phi = sqrt(n!) * P_* e. (6.7)
Claim 6.2a. ||phi|| = 1, and P_A^lambda phi = P_B^lambda phi.
Proof of the unit norm. The projection formula on diagonal
invariants is
P_* e = (1/n!) sum_{g in S_n} g e.
Pairing e against P_* e,
= (1/n!) sum_g
= (1/n!) sum_g
= (1/n!) sum_g f_R(g) f_F(g).
For g not in R the factor f_R(g) vanishes, and for g not in F the
factor f_F(g) vanishes. Since R cap F = {1} in S_n, the product
f_R(g) f_F(g) is zero for every g != 1, and is 1 at g = 1. Thus
= 1/n!. Because P_* is an orthogonal projection,
||P_* e||^2 = = 1/n!, and hence ||phi||^2 = n! * (1/n!)
= 1. []
Proof of agreement of projections. On the diagonal invariants, the
A-action of g equals the B-action of g^{-1}. The two sides therefore
have the same character values, and the two projection operators
agree on this subspace. []
We now compute ||P_A^lambda phi||^2. Expand the two averaging
formulas
P_A^lambda = (D_lambda / n!) sum_{g in S_n} chi_lambda(g^{-1}) rho_A(g),
P_* = (1 / n!) sum_{g in S_n} rho(g),
where rho_A is the A-side action and rho is the diagonal action.
Insert these into ||P_A^lambda phi||^2 = , use
the factorizations (6.6) (f_R vanishes outside R and f_F vanishes
outside F), and note that R cap F = {1}. Only the pairs
(z, x) in R x F survive:
||P_A^lambda phi||^2
= (D_lambda / n!) sum_{x in F, z in R}
chi_lambda(z^{-1} x) f_R(z) f_F(x). (6.8)
Both f_R and f_F are real and unchanged by inversion (v_R and v_F are
real unit vectors). Consequently the group-algebra coefficients of
E_R (as an element of the group algebra of S_n) are (D_R / |R|) f_R,
and similarly for E_F. Multiply the trace of the product of the two
group-algebra elements by D_lambda / n! * |R| |F| / (D_R D_F),
identity (6.8) becomes
||P_A^lambda phi||^2
= |R| |F| / (D_R D_F n!) * D_lambda Tr_{V_lambda}(E_R E_F). (6.9)
This is the normalization we will invert at the end of the proof.
>> In plain words. Tr(E_R E_F) is a codimension-2 quantity: it is
>> how much the row-Fourier projection "comes from" the column-Fourier
>> projection inside V_lambda. Equation (6.9) changes variables to
>> the squared norm of a unit vector after a projection to the
>> lambda-component. Now we will bound that squared norm in two
>> ways: from below via retention, from above via compression.
6.3 Random-plane retention: lambda-component is not lost
----------------------------------------------------------------
Recall H = a r, the dimension of the good even (and of the good odd)
space of the alphabet A. Choose an independent, unitarily invariant
complex r-plane in each of those two spaces, and set
V_A = (r-plane in even good) (+) (r-plane in odd good), (6.10)
so V_A has complex dimension 2r and sits inside A (x) C.
Let T_A be the spectral projector onto the vectors in A^{otimes n}
(x) C that have at most L_A slots outside V_A, where
L_A = 3 s + 2 r H. (6.11)
Every slot that carries an exceptional letter counts as outside V_A.
Level-A bad count: we allow the "outside" contribution to be at most
2 r H slots for the good letters not landing in the chosen r-plane
(complement of an r-plane inside an H-dim space has dimension H - r,
so we pay rH per parity). The 3s allowance breaks into
h_A + 2s: h_A for the exceptional slots (h_A <= s by (6.5)), and
2s for the Schur-Weyl complement counts of rho and xi (each <= s + rH).
So
h_A + 2 (s + rH) <= s + 2s + 2rH = 3s + 2rH = L_A.
Write overline T_A for the average of T_A over the random r-planes.
Lemma 6.3 (Plane retention). On the sector with exactly h_A
exceptional slots,
overline T_A >= alpha_H P_A^lambda,
alpha_H = (n+H)^{-2 r H}. (6.12)
The inequality holds on all multiplicity spaces.
Proof. Fix also the counts k, m of good even and good odd slots.
Apply ordinary Schur-Weyl separately to these two species. For
unitary highest weights rho of size k and xi of size m, the
symmetric-group side is induced from the Specht representations of
rho, xi' (transpose), and the exceptional tensor representation; the
arrangements of the three species among the slots give the induced
representation. If this summand contains lambda, then branching and
reciprocity give
rho <= lambda, xi' <= lambda (in Young order). (6.13)
The highest weights rho and xi have height at most H (the number of
rows in the unitary group). Therefore both rho and xi have at most
s + rH boxes strictly below the first r rows: for rho, at most rH of
those boxes lie within the first r columns, and the rest are counted
by s(lambda); for xi, use s(lambda') = s(lambda).
For fixed r-planes, the two complement-counts (even and odd) are
Lie-algebra operators on the two unitary factors. So the test T_A
is a spectral projection of the sum of these two counts, plus
h_A * I. On multiplicity spaces, it acts as the identity even
before averaging. Its average is scalar on each pair of unitary
types, with scalar equal to its value for reference planes.
For one highest weight nu, replace the r-plane inside the H-dim
unitary space by the first r coordinate directions. The HW vector
of nu has complement count sum_{i > r} nu_i. The U(H-r)-rep
generated from it has the same complement count (the complement-count
operator is central for U(H-r)); its highest weight is
(nu_{r+1}, ..., nu_H). Weyl's dimension formula gives its dimension
relative to the full U(H)-rep as
(prod_{r < i < j <= H} ( 1 + (nu_i - nu_j)/(j - i) ))
/ (prod_{1 <= i < j <= H} ( 1 + (nu_i - nu_j)/(j - i) ))
>= (n+H)^{-rH}. (6.14)
On the product of the two such subrepresentations (even and odd), the
test succeeds because h_A + 2 (s + rH) <= L_A. The resulting relative
trace is at least alpha_H, proving the lemma. []
Use the column side analogously: H' = b r, L_B = 3 s + 2 r H', and
corresponding projector T_B. Diagonal averaging preserves the
exceptional counts, so phi lies in both sectors to which Lemma 6.3
applies. The two positive operator inequalities tensor together.
Because P_A^lambda P_B^lambda phi = P_A^lambda phi (both projections
act the same on phi), we get
alpha_H alpha_H' ||P_A^lambda phi||^2
<= E || (T_A (x) T_B) phi ||^2. (6.15)
This is the lower bound for the tested norm. We now obtain its upper
bound, for every fixed choice of planes, in the next subsection.
>> In plain words. The random planes retain at least a fraction
>> alpha_H alpha_H' ~ exp(-O(n^{0.52})) of the lambda-component. So
>> fixing the planes does not lose too much of phi.
6.4 Cell-tag compression: the tested norm is tiny
----------------------------------------------------------------
Regroup the two alphabets into D^{otimes n}, where D = A (x) B has
parity equal to the sum of the two parities. The identification is
unitary and uses graded signs (moving a B-letter past a later-slot
A-letter contributes a minus sign when both are odd). Checking
adjacent transpositions shows that the diagonal action becomes the
graded permutation action on D. The parity-preserving slot tests
are identified without additional signs.
The slot in cell (i,j) now has tag
D_{ij} = A_i (x) B_j. (6.16)
These n tag spaces are mutually orthogonal, and e lies in their
product tag subspace. We do not (and do not need to) assert any
factorization of e across the slots.
Passing both tests leaves at most L_A + L_B slots outside W = V_A
(x) V_B. The tests preserve diagonal invariants. In an invariant
graded tensor, the number of slots in the odd part of W is at most
its dimension: after fixing their positions and grouping them,
those factors alternate. Let W_0 be the even part of W, and let
Delta_* be the test that at most
T = L_A + L_B + 4 r^2 (6.17)
slots lie outside W_0. We obtain
|| (T_A (x) T_B) phi ||^2 <= || Delta_* phi ||^2
= n! || P_* Delta_* e ||^2. (6.18)
The equality uses the commutation of Delta_* with the diagonal
action (Delta_* is a slot-content projector and hence diagonal-
invariant) and the definition phi = sqrt(n!) P_* e:
||Delta_* phi||^2 = n! ||Delta_* P_* e||^2 = n! ||P_* Delta_* e||^2.
Also
dim W_0 <= 4 r^2, T <= C X / log n < n/2 (6.19)
for large n: the terms in T not involving s are O(n^{0.52}), and
the lower endpoint of (6.1) absorbs them.
Lemma 6.4 (Cell-tag compression). With the notation above, put
M_* = sum_{t <= T} binom(n, t). (6.20)
Then
n! || P_* Delta_* e ||^2
<= n! * M_*^2 * max_{0 <= t <= T}
dim(Sym^{n-t} W_0) / (t! (n-t)^{n-t}). (6.21)
Proof. Write Delta_* = sum_{|S| <= T} Delta_S, where Delta_S
specifies exactly the slots outside W_0 (as a graded slot-content
projector). Fix S, put t = |S|, m = n - t, and group the remaining
slots before those in S, using graded reordering. Parity-stability
of the tags ensures that the reordered vector still lies in the
corresponding product tag subspace.
The full invariant projection P_* is the projection onto symmetric
n-tensors of D, and S_m x S_t is a subgroup of S_n. Therefore
P_* is dominated, in the Loewner order, by the product of the
"within-group symmetric" projections
P_* <= P_{S_m-symm} (x) P_{S_t-symm}.
Since Delta_S commutes with those two projections, and the vector
Delta_S e lives in W_0^{otimes m} (x) D^{otimes t}, we may replace
each factor by its tight version on the vector's support:
|| P_* Delta_S e ||^2 <= quadratic form of
Pi_m (x) Pi_t ,
Pi_m = P_{Sym^m W_0}, Pi_t = P_{graded symmetric in D^{otimes t}}. (6.22)
We now compress this operator to the product tag subspace. The
compression splits into the tensor product of the two compressed
operators: no tag projection is required to commute with P_{W_0}.
On the S-slots every nonidentity permutation changes a tag, so the
compression of Pi_t is exactly identity divided by t!.
For the other factor, unitary invariance and irreducibility give
Pi_m = dim(Sym^m W_0) * E_z |z^{otimes m}>> In plain words. Each permutation assignment S contributes only a
>> tiny amount because the tags force the "spread" of e. Bounding
>> how many assignments there are gives M_*; Cauchy-Schwarz ensures
>> we only pay the square of M_*.
6.5 Finishing the proof of Proposition 6.1
----------------------------------------------------------------
The number of assignments is at most
M_* <= exp( C T log(en/T) ). (6.25)
Use dim W_0 <= 4 r^2 and Stirling in the compression lemma. In
particular, for each t,
n! / (t! (n-t)^{n-t})
= binom(n, t) exp( -(n-t) + O(log n) ). (6.26)
We get
E || (T_A (x) T_B) phi ||^2
<= exp( -n + O( T log(en/T) + r^2 log n ) ). (6.27)
The inverse retention factor in (6.15) costs
log( 1 / (alpha_H alpha_H') ) = O(n^{0.6}). (6.28)
Finally, (6.9) restores the subgroup normalization:
D_lambda Tr(E_R E_F) / (D_R D_F)
= ||P_A^lambda phi||^2 * n! / (|R||F|). (6.29)
By Stirling,
n! / (|R||F|) <= exp( n + O(sqrt(n) log n) ). (6.30)
Combining (6.27), (6.28), (6.30), the compression factor -n from
(6.27) cancels the +n from (6.30) exactly. What remains is
D_lambda Tr(E_R E_F) / (D_R D_F)
<= exp( O( T log(en/T) + r^2 log n + n^{0.6}
+ sqrt(n) log n ) ). (6.31)
Now use the dimension window (6.1) and T <= C X/log n. The bound
T log(en/T) <= O( X / sqrt(log n) ) (6.32)
is proved as follows. The function T |-> T log(en/T) is increasing
for 0 < T < n/e, and T = T_max <= CX/log n << n/e for large n. So
its maximum over 0 <= T <= T_max is at T = T_max. At that point,
log(en/T_max) = log(en) - log(T_max) = log(en) - log(CX/log n)
= O( sqrt(log n) ) using X >= n e^{-A_0 sqrt(log n)}. Hence
T_max log(en/T_max) = O( X / sqrt(log n) ).
The remaining terms on the right of (6.31) are absorbed by the
dimension window. Indeed, r^2 log n = O(n^{0.02} log n) = o(n);
n^{0.6} = o(n); sqrt(n) log n = o(n); and X >= n e^{-A_0 sqrt(log n)}
satisfies X = n^{1-o(1)}, so each of these terms is
O(X / sqrt(log n)) for large n.
Therefore, for sufficiently large n,
D_lambda Tr(E_R E_F) / (D_R D_F)
<= exp( O( X / sqrt(log n) ) ). (6.33)
Since sqrt(log n) is much larger than (log n)^{1/3} for large n,
this is at most
exp( X / (log n)^{1/3} ),
which is (6.2). This proves Proposition 6.1. []
7. The uniform moment induction
----------------------------------------------------------------
Now we assemble the proof of Theorem A. The induction is on the bit
length d = log_2 n. Conclusion: Tr_reg|Q_n|^P <= 1 + n^{-10}.
At each dyadic n we choose an exponent P(n), prove (2.1) at
P = P(n), and afterwards show P(n) is uniformly bounded in n.
7.1 Covering the representation ranges
----------------------------------------------------------------
Throughout X = log D_lambda.
Proposition 7.1 (Ranges covered). There exist absolute constants
A_0, A_1 > 0 and P_c >= 2 such that, for all sufficiently large
dyadic n,
sum over lambda partition of n, lambda != trivial, such that
X >= A_1 n or 1 <= n - lambda_1 <= n e^{-M sqrt(log n)}
of D_lambda Tr_{V_lambda} |Q_n(lambda)|^{P_c}
<= n^{-12}. (7.1)
Here M is the constant from Proposition 5.1.
Moreover, every nontrivial lambda not counted in (7.1) and not of
height > n/2 satisfies
n e^{-A_0 sqrt(log n)} <= X <= A_1 n. (7.2)
Proof of (7.1).
(i) Annihilation for height > n/2.
A coordinate matching average is the operator
Q_match = (1 / 2^{n/2}) sum_{g in (S_2)^{n/2}} g on the
n cards, where (S_2)^{n/2} is the elementary abelian subgroup
swapping two halves along a fixed coordinate. Its image is
the permutation module on perfect matchings, whose irreducible
components by Pieri are exactly the partitions of height <= n/2.
Since each of the d coordinate matchings appears once in the
sweep Q_n, the same vanishing holds for Q_n(lambda). So lambdas
of height > n/2 contribute zero.
(ii) Heavy dimension.
Apply Proposition 4.1 at power 2u. Then
Tr_{V_lambda} |Q_n(lambda)|^{2u} * D_lambda is exactly the
lambda-contribution to Tr_reg |Q_n|^{2u} <= e^{Cn}. So
Tr_{V_lambda} |Q_n(lambda)|^{2u} <= e^{Cn} / D_lambda.
For p' >= 2u, use sum_j a_j^t <= (sum_j a_j)^t:
Tr_{V_lambda} |Q_n(lambda)|^{p'}
<= (e^{Cn} / D_lambda)^{p'/(2u)}.
Set A_1 = 2 C and p' >= 12 u. Then on X >= A_1 n,
Tr_{V_lambda} |Q_n(lambda)|^{p'}
<= D_lambda^{-p'/(4u)} <= D_lambda^{-3}.
So
sum over X >= A_1 n D_lambda Tr |Q_n(lambda)|^{p'}
<= sum over those lambdas D_lambda^{-2}
<= e^{O(sqrt n)} * e^{-2 A_1 n} [D_lambda >= e^{A_1 n}]
<= n^{-12} (7.3)
for large n. Uses: #partitions of n is at most e^{O(sqrt n)}
(evaluating prod_l (1 - x^l)^{-1} at x = e^{-1/sqrt n}).
(iii) Sparse rows.
Directly Proposition 5.1 with exponent P_s. Its contribution is
at most n^{-12}.
Since all singular values of Q_n(lambda) are at most 1, raising
P_s to P_c = max{P_s, 12u} can only decrease the sum. So (7.1)
holds with P_c = max{P_s, 12u}. []
Proof that every remaining lambda has X >= ne^{-A_0 sqrt(log n)}.
We assume lambda is not covered by (7.1): not trivial, not in the
sparse range, not of height > n/2. Also X < A_1 n (else (7.1)
applies).
Case A: first row or first column has length L >= n/8.
Transpose if necessary; D_lambda is invariant under transpose.
Retain the first row together with a subdiagram of
t = floor(n e^{-M sqrt(log n)})
boxes below it. There are enough boxes: in the row case we
excluded the sparse range, so the tail below the first row has
at least n e^{-M sqrt(log n)} >= t boxes; in the column case
the original height is <= n/2, so the transposed tail has at
least n/2 >> t boxes.
Since t = o(L) (L >= n/8 >> t), the hook comparison (5.10)
applied to this smaller diagram gives log D_lambda >= t for
large n. Branching bounds the smaller diagram's dimension by
D_lambda. Hence
X >= t >= n e^{-(M+1) sqrt(log n)}.
So set A_0 = M + 1.
Case B: both first row and first column have length < n/8.
Every hook length in lambda is at most n/4 (first row plus first
column), and the hook formula gives
D_lambda >= n! / (n/4)^n,
log D_lambda >= c n
for an absolute c > 0, by Stirling. Choose A_0 also >= 1;
then X >= c n >> n e^{-A_0 sqrt(log n)} for large n. []
7.2 Closing the induction
----------------------------------------------------------------
Proof of (2.1) in Theorem A.
Base case. Fix a threshold n_0 large enough for all the preceding
estimates. For each of the finitely many dyadic sizes 2 <= n < n_0,
every nonconstant "regular singular value" of Q_n is strictly less
than one. [Why? Equality would force a common invariant vector for
all coordinate switches; those generate S_n, so only constants can
have equality.]
Choose a common finite power P(n) >= P_c large enough (there are
finitely many singular values to handle) so that
Tr_reg|Q_n|^{P(n)} <= 1 + n^{-10}
holds for all dyadic n < n_0.
Inductive step. Let n = 2^d with d > log_2 n_0. Split the d
coordinates into consecutive groups of sizes floor(d/2) and
ceil(d/2). This induces a balanced (a, b) grid:
a = 2^{floor(d/2)}, b = 2^{ceil(d/2)}, ab = n,
sqrt(n)/2 <= a <= sqrt(n) <= b <= 2 sqrt(n). (7.4)
In matrix-product order Q_n = YZ: Y is the "row law"
(independent b-card cube sweeps on the rows), Z is the "column law"
(independent a-card cube sweeps on the columns).
Define
p = max{ P(a), P(b), P_c },
P(n) = (1 + 2 (log n)^{-1/4}) p. (7.5)
The already controlled classes (from Proposition 7.1) retain
their bounds at this larger power, since raising an exponent by a
factor t >= 1 keeps values <= 1; on X >= A_1 n and on the sparse
rows this is exactly what (7.3) requires (apply (7.3) at exponent
P_c, then raise).
Consider a remaining representation lambda, with X in (7.2).
Araki-Lieb-Thirring. For positive matrices B, C and exponent
r >= 1,
Tr |B C|^r <= Tr (B^* B)^{r/2} (C C^*)^{r/2}. (7.6)
Apply with B = Y(lambda), C = Z(lambda) and r = p/2 >= 1. The
left side is exactly Tr_{V_lambda} |Q_n(lambda)|^p.
Take spectral decompositions of the two positive powers
(Y^*Y)^{p/2} and (ZZ^*)^{p/2} on their own subgroups irreducibles,
then restrict to V_lambda. Because the models are real orthogonal,
real eigenvectors exist; so the resulting projections are
*exactly* the E_R, E_F controlled by Proposition 6.1. Each such
projection acts on all copies of its carrier, with a nonnegative
spectral coefficient.
For either subgroup, the sum of those coefficients times the carrier
dimension is its regular Schatten-p moment on the corresponding
subgroup. Product subgroup laws are independent across lines, so
those moments factor:
row side: (1 + b^{-10})^a (induction hypothesis at size b);
column side: (1 + a^{-10})^b (induction hypothesis at size a).
Multiplying (7.6) by D_lambda and applying Proposition 6.1 and the
induction hypothesis:
D_lambda Tr_{V_lambda} |Q_n(lambda)|^p
<= exp( X / (log n)^{1/3} )
* (1 + b^{-10})^a * (1 + a^{-10})^b. (7.7)
The last two factors are bounded by an absolute constant
(concretely, (1 + b^{-10})^a <= exp(a/b^{10}) <= exp(2) for a,b as
in (7.4) with n large). The lower endpoint in (7.2)
n e^{-A_0 sqrt(log n)} <= X absorbs that constant. Hence
Tr_{V_lambda} |Q_n(lambda)|^p
<= exp(-X + X / (log n)^{1/3}). (7.8)
Raise to the power t = P(n)/p = 1 + 2 (log n)^{-1/4}:
sum_j a_j^t <= (sum_j a_j)^t for a_j >= 0, t >= 1.
So
D_lambda Tr_{V_lambda} |Q_n(lambda)|^{P(n)}
<= ( D_lambda Tr_{V_lambda} |Q_n(lambda)|^p )^t
<= exp( -t X + t X / (log n)^{1/3} ).
The exponent is
-t X + t X / (log n)^{1/3}
= -X * t * ( 1 - (log n)^{-1/3} )
= -X * ( 1 + 2 (log n)^{-1/4} ) ( 1 - (log n)^{-1/3} )
= -X * ( 1 + 2 (log n)^{-1/4} - (log n)^{-1/3} + O((log n)^{-7/12}) ).
For large n this bracket is at least 1 + c (log n)^{-1/4} for an
absolute c > 0, since 2 (log n)^{-1/4} > (log n)^{-1/3}. So
D_lambda Tr_{V_lambda} |Q_n(lambda)|^{P(n)}
<= exp( -c X / (log n)^{1/4} ). (7.9)
Summability. Using the lower bound (7.2) on X and Proposition 7.1
(finitely many pieces), we estimate the total over all remaining
nontrivial lambdas:
sum over remaining lambda D_lambda Tr |Q_n(lambda)|^{P(n)}
<= sum over e^{O(sqrt n)} partitions
exp( -c n e^{-A_0 sqrt(log n)} / (log n)^{1/4} )
<= n^{-12} (7.10)
for sufficiently large n.
Total. Add (7.1), (7.10), and the trivial contribution (= 1):
Tr_reg|Q_n|^{P(n)} <= 1 + 3 n^{-12} <= 1 + n^{-10}. (7.11)
Uniformity of P(n). Along any branch of the recursion
d -> floor(d/2), ceil(d/2) down to the base range, the bit lengths
grow geometrically when read in reverse: at level k they are
approximately d_0 * 2^k. So (log n_k)^{-1/4} ~ (d_0 * 2^k)^{-1/4},
and sum_k 2 (log n_k)^{-1/4} <= C_0 sum_k 2^{-k/4} < infinity.
Therefore the product of the factors (1 + 2 (log n_k)^{-1/4}) over
k is uniformly bounded, and P(n) <= P for a fixed absolute P
independent of n. This proves (2.1) at this fixed P.
7.3 From traces to total variation
----------------------------------------------------------------
We finish the second half of Theorem A: for any integer v with
2v >= P,
|| mu_v - U_{S_n} ||_TV <= (1/2) n^{-5}.
Recall mu_v = Q_n^v as a probability distribution (Q_n convolved with
itself v times, i.e. v independent sweeps), and Pi = projection onto
constants in the regular representation. In the regular
representation on l^2(S_n), Pi has constant matrix 1/n!.
Step 1: Schatten exponent. For any square matrix A,
|A^v|^2 = (A^v)^* A^v, and a cyclic trace gives
Tr |A^v|^2 = Tr (A^*)^{v} A^{v} = Tr (A^* A)^{v} = Tr |A|^{2v}.
Here the second equality uses cyclicity of trace repeatedly and the
third is the definition of |A| = (A^* A)^{1/2}. Applying with
A = Q_n and restricting to the nonconstant part,
Tr_{reg, perp} |Q_n^v|^2 = Tr_{reg, perp} |Q_n|^{2v}. (7.12)
Step 2: Application of (2.1). For integer v with 2v >= P,
Tr_{reg, perp} |Q_n|^{2v}
<= Tr_{reg, perp} |Q_n|^P (2v >= P and singular values <= 1)
<= n^{-10}. (7.13)
Step 3: Regular trace equals Hilbert-Schmidt on the nonconstant
part. Because Q_n^v(triv) = 1 and Pi(triv) = 1, the difference
Q_n^v - Pi has zero constant part. Hence
|| Q_n^v - Pi ||_{HS}^2 = Tr_{reg, perp} |Q_n^v|^2
<= n^{-10}. (7.14)
Step 4: Total variation. Every row of the regular transition
matrix Q_n^v is a translate of mu_v; more precisely, Q_n^v as a
matrix indexed by (g, h) in S_n x S_n has entry
Q_n^v(g, h) = mu_v(g^{-1} h), (7.15)
and Pi(g, h) = 1/n!. Therefore
|| Q_n^v - Pi ||_{HS}^2
= sum_{g, h} ( mu_v(g^{-1} h) - 1/n! )^2
= n! * sum_{g} ( mu_v(g) - 1/n! )^2, (7.16)
because for each fixed g the substitution h -> g^{-1} h sweeps
S_n once.
By Cauchy-Schwarz,
|| mu_v - U_{S_n} ||_TV
= (1/2) sum_g |mu_v(g) - 1/n!|
<= (1/2) sqrt( n! * sum_g (mu_v(g) - 1/n!)^2 )
= (1/2) || Q_n^v - Pi ||_{HS}
<= (1/2) n^{-5}.
This is uniform over the initial deck: relabeling g -> g x^{-1}
translates mu_v to the version started from deck x, and the TV
distance to uniform is unaffected by relabeling. This completes
the proof of Theorem A. []
8. The matching lower bound
----------------------------------------------------------------
Here we show that O(d) physical shuffles is optimal in order:
mixing to total variation 1/4 needs at least 2d - O(1) physical
shuffles. By contrast, one full coordinate sweep is d physical
shuffles, so the theorem gives mixing in O(d) physical shuffles.
Proposition 8.1 (Support obstruction). After t physical shuffles
(that is, t/2 sweeps), the support of the resulting distribution
mu_t is a subset of S_n of size at most
2^{tn/2}. (8.1)
Consequently, for any t,
|| mu_t - U_{S_n} ||_TV >= 1 - 2^{tn/2} / n!. (8.2)
Setting the bound (8.2) equal to 1/4 gives the stated requirement:
t >= ceil( (2/n) log_2(3 n! / 4) ) = 2d - O(1). (8.3)
Proof. Count coin strings. A physical shuffle uses n/2 independent
fair bits (one per pair across a coordinate). So t physical
shuffles jointly use tn/2 bits, hence produce at most 2^{tn/2}
distinct permutations. [One might worry that some coin strings
produce the same permutation; that only shrinks the support.]
So |supp mu_t| <= 2^{tn/2}.
Lower-bound TV test against uniform. Let A = supp mu_t. Since
mu_t(A) = 1 and U_{S_n}(A) = |A|/n!, the TV distance -- which is the
supremum over subsets E of |mu_t(E) - U_{S_n}(E)| -- dominates
|mu_t(A) - U_{S_n}(A)| = 1 - |A|/n!.
So (8.2) holds.
Solve for t. For TV <= 1/4 (mixing), we require
2^{tn/2} / n! >= 3/4, i.e.
tn/2 >= log_2(3 n! / 4), i.e. t >= (2/n) log_2(3 n! / 4).
With Stirling's formula n! ~ (n/e)^n sqrt(2 pi n),
(2/n) log_2(3 n! / 4)
~ (2/n) * (n log_2 n - n log_2 e + log_2(3 sqrt(2 pi n) / 4))
= 2 log_2 n - 2 log_2 e + O(log n / n)
= 2d - 2 log_2 e + o(1).
So t >= 2d - O(1). []
>> In plain words. A physical shuffle has too few coins to reach
>> many permutations. To even come close to uniformity on S_n (which
>> has n! elements), you need enough physical shuffles that
>> 2^{tn/2} is comparable to n!, which requires tn/2 ~ n d, i.e.
>> t ~ 2d. So the O(d) in the main theorem is the right order.
9. References
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