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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].
The affine Hecke algebra at rank one is in the nondegenerate normalization and in the degenerate normalization, and the cyclotomic quotient is obtained by imposing the first-variable cyclotomic relation (Affine and cyclotomic type-A Hecke algebras).
The cyclotomic quotient has a bounded spanning family and finite dimension at most (Basis and dimension of the cyclotomic Hecke algebra).
The nonzero central sectors are the blocks, with the level-one Jucys–Murphy identification (Residue-sequence idempotents and cyclotomic Hecke blocks).
The blockwise maps are mutually inverse isomorphisms for every content with (The Brundan–Kleshchev blockwise Hecke–KLR isomorphism).
Counterexample
Given: The presentation and block conventions of [F1]–[F4].
The positive-rank affine algebra is infinite dimensional. At rank one the affine presentation is in NG and in DG by [F1], and their distinct powers are independent. More generally for any , the NG assignments , give a surjection onto : the quadratic relation reads , the Bernstein relation reads , and the commuting and braid relations are scalar identities. In DG the assignments , similarly give a surjection onto , since the adjacent mixed relation reads and all other relations hold. The images contain , 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 , and the obstruction does not apply.
The whole cyclotomic quotient is not one block. Take (level one), , and a primitive cube root of unity. By the level-one identification [F3] the algebra is , since in characteristic . Here and , so the two factors and have polynomial eigenwords and , hence residue words and . Thus there are two nonzero blocks and , each one dimensional with residue sequences and . Therefore has dimension and is not isomorphic to either one-dimensional block; on the KLR side [F4] gives , which is not isomorphic to a single block.
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 ; 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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Sources
- Jonathan Brundan and Alexander Kleshchev, Blocks of cyclotomic Hecke algebras and Khovanov–Lauda algebras, arXiv:0808.2032v3, Invent. Math. 178 (2009) 451–484 (complete 32-page article) (standard reference, not scraped)
- Jun Hu and Andrew Mathas, Seminormal forms and cyclotomic quiver Hecke algebras of type A, arXiv:1304.0906v3 (published in Math. Ann.), complete 51-page article (standard reference, not scraped)