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At positive rank the whole affine Hecke algebra is not one cyclotomic KLR block

Statement refuted

One might hope that for a fixed content-block isomorphism, replacing one side by the whole affine Hecke algebra, or by the whole cyclotomic quotient in the example below, still gives an isomorphism. This fails. The failure does not forbid summing all the central-sector isomorphisms on both sides.

Facts & Assumptions

Given: The affine and cyclotomic presentation conventions [F1], the bounded basis and dimension of the cyclotomic quotient [F2], the residue-projector/block conventions [F3], and the blockwise isomorphism [F4].

[F1]

The affine Hecke algebra at rank one is F[X1±1] in the nondegenerate normalization and F[x1] in the degenerate normalization, and the cyclotomic quotient is obtained by imposing the first-variable cyclotomic relation (Affine and cyclotomic type-A Hecke algebras).

[F2]

The cyclotomic quotient has a bounded spanning family and finite dimension at most ℓdd! (Basis and dimension of the cyclotomic Hecke algebra).

[F3]

The nonzero central sectors HαΛ=eαHdΛ are the blocks, with the level-one Jucys–Murphy identification HdΛ≅HF,q(Sd) (Residue-sequence idempotents and cyclotomic Hecke blocks).

[F4]

The blockwise maps ρ ⁣:RαΛ→HαΛ are mutually inverse isomorphisms for every content with eα≠0 (The Brundan–Kleshchev blockwise Hecke–KLR isomorphism).

Counterexample

technique · direct

Given: The presentation and block conventions of [F1]–[F4].

1.1F1F2F4algebra

The positive-rank affine algebra is infinite dimensional. At rank one the affine presentation is F[X1±1] in NG and F[x1] in DG by [F1], and their distinct powers are independent. More generally for any d≥1, the NG assignments Tr↦qH, Xr↦qHr−1t give a surjection onto F[t,t−1]: the quadratic relation reads qH2=(qH−1)qH+qH, the Bernstein relation reads qH2qHr−1t=qHqHrt, and the commuting and braid relations are scalar identities. In DG the assignments sr↦1, xr↦t+(r−1)1F similarly give a surjection onto F[t], since the adjacent mixed relation reads t+(r−1)=(t+r)−1 and all other relations hold. The images contain t, so the affine algebras are infinite-dimensional at every positive rank. Every cyclotomic block is finite-dimensional by [F2], and its KLR partner has the same dimension by [F4]; thus neither is isomorphic to the whole positive-rank affine algebra. At rank zero both presentations give F, and the obstruction does not apply.

2.1F3F4step 1.1

The whole cyclotomic quotient is not one block. Take Λ=Λ0 (level one), d=2, e=3 and qH a primitive cube root of unity. By the level-one identification [F3] the algebra is HF,q(S2)≅F[T]/((T−qH)(T+1))≅F×F, since qH≠−1 in characteristic ≠3. Here X1=1 and X2=qH−1T12, so the two factors T1=qH and T1=−1 have polynomial eigenwords (1,qH) and (1,qH2), hence residue words (0,1) and (0,2). Thus there are two nonzero blocks Hα0+α1Λ and Hα0+α2Λ, each one dimensional with residue sequences (0,1) and (0,2). Therefore H2Λ has dimension 2 and is not isomorphic to either one-dimensional block; on the KLR side [F4] gives R2Λ=Rα0+α1Λ⊕Rα0+α2Λ, which is not isomorphic to a single block.

3.1F1F2step 1.1step 2.1∎

Conclusion. Both branches of the attempted replacement fail: the affine algebra is infinite dimensional, and the whole cyclotomic quotient decomposes into more than one block. Summing the central-sector isomorphisms on both sides remains valid.

Remarks

At rank zero the affine algebra and the unique cyclotomic KLR content algebra are both F; there is no dimension obstruction or counterexample at that boundary. The counterexample separates the blockwise statement of the Brundan–Kleshchev theorem from two stronger readings: the rank-one affine algebra itself is not a cyclotomic block, and a cyclotomic quotient can have several blocks. No choice principle is used.

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