diff ab2b37b..cda06a4 artin_parabolic_intersections_v2.txt
diff --git a/artin_parabolic_intersections_v2.txt b/artin_parabolic_intersections_v2.txt
index 881fcf0..1de3b45 100644
--- a/artin_parabolic_intersections_v2.txt
+++ b/artin_parabolic_intersections_v2.txt
@@ -681,7 +681,7 @@ to f). So D = X and z = 1, giving
PROOF OF THE MAIN THEOREM.
- STEP 1. Set x = h^{-1} g in A (so g = h x^{-1}). Then
+ STEP 1. Set x = h^{-1} g in A (so g = h x). Then
g A_X g^{-1} ∩ h A_Y h^{-1}
= h ( x A_X x^{-1} ∩ A_Y ) h^{-1}.
@@ -954,21 +954,11 @@ EXAMPLE 5: a nonspherical case with braid relations
Take P = <a, b> and Q = <b, c>. Both are Braid_3-type. They
share <b>. Is the intersection bigger than <b>?
- Suppose x ∈ <a, b> ∩ <b, c> is not in <b>. Then x is a
- nonempty reduced word in {a, b, a^{-1}, b^{-1}} (in the first
- subgroup) that is also a reduced word in {b, c, b^{-1}, c^{-1}}
- (in the second). In the free product structure underlying A, an
- element is uniquely determined by its normal form. An element
- in the subgroup generated by a, b has normal form using only
- a-appearances and b-appearances; an element in the subgroup
- generated by b, c has normal form using only b-appearances and
- c-appearances. An element in BOTH must use only b-appearances
- (else it would have an a or c appearance, contradicting
- membership in the other subgroup; the subgroup <a, b> really
- contains elements with an "a" in the normal form, and similarly
- for c). So x ∈ <b>. Therefore:
-
- P ∩ Q = <b>.
+ By van der Lek's 1983 theorem (for standard parabolics defined
+ by subdiagrams), A_{X} ∩ A_{Y} = A_{X ∩ Y} for any X, Y ⊆ S.
+ Here X = {a, b}, Y = {b, c}, X ∩ Y = {b}. So:
+
+ P ∩ Q = A_{a,b} ∩ A_{b,c} = A_b = <b>.
Parabolic (rank 1). GOOD