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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: , , , , where
At the antispherical value is while the spherical value is , 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 .
Facts & Assumptions
Given: with simple reflections , the parabolic , the modules and coefficients of The parabolic Kazhdan–Lusztig bases and polynomials, (Standard parabolic subgroups, descent-free one- and two-sided representatives, parabolic and reflection subgroups, The parabolic Hecke module and the antispherical standard basis).
Ordinary Kazhdan--Lusztig coefficients: unless , , and for one has with terms of strictly smaller degree and (Existence and uniqueness of the Kazhdan–Lusztig basis); in the values needed below all have length gap at most two, for which the degree and parity bounds force .
Bruhat comparabilities in : for all , , , , , , , , while and are incomparable, and are incomparable, and is the maximum (Basic properties of the Bruhat order on ).
Antispherical alternating sum: for , with (Parabolic Kazhdan–Lusztig polynomials are alternating sums of ordinary ones (1), Existence and uniqueness of the parabolic Kazhdan–Lusztig bases).
Spherical reduction: when is finite with longest element ; here (Parabolic Kazhdan–Lusztig polynomials are alternating sums of ordinary ones (2)).
Counterexample
For and the antispherical alternating sum [F3] has the two terms and : . By [F1] the first value has length gap two, so ; the second has gap one (by [F2], ), so . Hence .
For the same pair the spherical reduction [F4] gives ; the pair has length gap two by [F2] (), so by [F1] and .
Specializing at gives : no unit of (indeed no unit of ) multiplied by gives , so the antispherical value is not a unit multiple of the spherical one; and since the two elements and of are distinct, they differ as polynomials and not only after specialization.
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 by the antispherical value produces the false multiplicity 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.
Depends on
- The parabolic Kazhdan–Lusztig bases and polynomials
- Parabolic Kazhdan–Lusztig polynomials are alternating sums of ordinary ones
- Existence and uniqueness of the parabolic Kazhdan–Lusztig bases
- The parabolic Hecke module and the antispherical standard basis
- Existence and uniqueness of the Kazhdan–Lusztig basis
- Kazhdan–Lusztig polynomials in the classical $q$-normalization
- The normalized type-A Hecke algebra and its bar involution
- Standard parabolic subgroups, descent-free one- and two-sided representatives, parabolic and reflection subgroups
- Basic properties of the Bruhat order on $S_n$
Used by
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Sources
- Nicolas Libedinsky and Geordie Williamson, The anti-spherical category (author-hosted complete PDF of the article) (standard reference, not scraped)
- Anders Bjoerner and Francesco Brenti, Combinatorics of Coxeter Groups, Graduate Texts in Mathematics 231, Springer 2005 (author-hosted complete 370-page PDF) (standard reference, not scraped)