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Spherical and antispherical normalizations give different polynomials

Statement refuted

Refuted claim: the spherical and antispherical parabolic Kazhdan--Lusztig coefficients of The parabolic Kazhdan–Lusztig bases and polynomials agree (equivalently, either normalization may be substituted for the other in a parabolic or singular-block multiplicity statement).

The witness is the smallest nontrivial parabolic: W=S3, J={s1}, WJ={1,s1}, JW={e,s2,s2s1}, where

ne,s2s1=pe,s2s1−v ps1,s2s1=v2−v2=0≠me,s2s1=ps1,s1s2s1=ps1,w0=v2.

At v=1 the antispherical value is 0 while the spherical value is 1, so the antispherical number is not a unit multiple of the spherical one; as polynomials they differ as well, so the discrepancy is not an artifact of specializing at v=1.

Facts & Assumptions

Given: W=S3 with simple reflections s1,s2, the parabolic J={s1}, the modules MJ,NJ and coefficients my,x,ny,x of The parabolic Kazhdan–Lusztig bases and polynomials, JW={e,s2,s2s1} (Standard parabolic subgroups, descent-free one- and two-sided representatives, parabolic and reflection subgroups, The parabolic Hecke module and the antispherical standard basis).

[F1]

Ordinary Kazhdan--Lusztig coefficients: py,w=0 unless y≤w, pw,w=1, and for y<w one has py,w∈vZ[v] with py,w=vℓ(w)−ℓ(y)+ terms of strictly smaller degree and py,w∈vℓ(w)−ℓ(y)Z[v−2] (Existence and uniqueness of the Kazhdan–Lusztig basis); in S3 the values needed below all have length gap at most two, for which the degree and parity bounds force py,w=vℓ(w)−ℓ(y).

[F2]

Bruhat comparabilities in S3: e<w for all w≠e, s1<s1s2, s1<s2s1, s1<w0, s2<s2s1, s2<w0, s1s2<w0, s2s1<w0, while s1 and s2 are incomparable, s1s2 and s2s1 are incomparable, and w0 is the maximum (Basic properties of the Bruhat order on Sn).

[F3]

Antispherical alternating sum: ny,x=∑z∈WJ(−1)ℓ(z)vℓ(z)pzy,x for x,y∈JW, with ℓ(zy)=ℓ(z)+ℓ(y) (Parabolic Kazhdan–Lusztig polynomials are alternating sums of ordinary ones (1), Existence and uniqueness of the parabolic Kazhdan–Lusztig bases).

[F4]

Spherical reduction: my,x=pw0Jy,w0Jx when WJ is finite with longest element w0J; here w0J=s1 (Parabolic Kazhdan–Lusztig polynomials are alternating sums of ordinary ones (2)).

Counterexample

technique · direct two-term computation in the Hecke algebra of $S_3$
1.1F1F2F3algebra

For y=e and x=s2s1 the antispherical alternating sum [F3] has the two terms z=e and z=s1: ne,s2s1=pe,s2s1−v ps1,s2s1. By [F1] the first value has length gap two, so pe,s2s1=v2; the second has gap one (by [F2], s1<s2s1), so ps1,s2s1=v. Hence ne,s2s1=v2−v⋅v=0.

1.2F1F2F4algebra

For the same pair the spherical reduction [F4] gives me,s2s1=ps1e, s1s2s1=ps1,w0; the pair has length gap two by [F2] (ℓ(w0)−ℓ(s1)=3−1=2), so by [F1] ps1,w0=v2 and me,s2s1=v2.

2.1step 1.1step 1.2algebra

Specializing at v=1 gives ne,s2s1(1)=0≠1=me,s2s1(1): no unit of Z (indeed no unit of C) multiplied by 0 gives 1, so the antispherical value is not a unit multiple of the spherical one; and since the two elements 0 and v2 of Z[v] are distinct, they differ as polynomials and not only after specialization.

3.1step 2.1F3F4algebra∎

Consequently the two normalizations cannot be substituted for one another in a multiplicity statement: a singular-block or parabolic multiplicity theorem must fix the side of the parabolic and the corresponding module. In particular the singular-block multiplicity theorem of this page is stated in the spherical normalization, whereas a genuine regular parabolic locally finite category uses the antispherical family; replacing the spherical value me,s2s1(1)=1 by the antispherical value ne,s2s1(1)=0 produces the false multiplicity 0 for this pair. The computation refutes the normalization-blind claim, not the parabolic basis theorem or the alternating-sum identity that produced the two values.

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