The Broué–Malle–Michel cohomological conjecture for good elements

Let G\mathbf G be an algebraic group over Fˉq\bar{\mathbb F}_q, let FF be a Frobenius map, let G=GFG=\mathbf G^F, and let WW be the FF-split Weyl group. Let wWw\in W be a good element of order dd, let XwX_w be the associated Deligne–Lusztig variety, and fix a prime \ell not dividing qq. Write Hci(Xw,Qˉ)H^i_c(X_w,\bar{\mathbb Q}_\ell) for its étale cohomology, and let H(G,F,W,w)\mathcal H(\mathbf G,F,W,w) be the image of QˉBw\bar{\mathbb Q}_\ell B_w in the graded endomorphism algebra of i0Hci(Xw,Qˉ)\bigoplus_{i\geq0}H^i_c(X_w,\bar{\mathbb Q}_\ell). Broué–Malle–Michel's conjecture. Suppose that ww is a good element of order dd. Then: (1) if iji\ne j, the QˉGF\bar{\mathbb Q}_\ell\mathbf G^F-modules Hci(Xw,Qˉ)H^i_c(X_w,\bar{\mathbb Q}_\ell) and Hcj(Xw,Qˉ)H^j_c(X_w,\bar{\mathbb Q}_\ell) have no irreducible constituents in common; (2) there is a dd-cyclotomic Hecke algebra Hx(CW(w))\mathscr H_x(C_W(w)) such that

H(G,F,W,w)HQˉ,q(CW(w))EndQˉGF(i0Hci(Xw,Qˉ));\mathcal H(\mathbf G,F,W,w)\cong\mathscr H_{\bar{\mathbb Q}_\ell,q}(C_W(w))\cong\operatorname{End}_{\bar{\mathbb Q}_\ell\mathbf G^F}\left(\bigoplus_{i\geq0}H^i_c(X_w,\bar{\mathbb Q}_\ell)\right);

(3) there is a one-to-one correspondence χχq\chi\longleftrightarrow\chi_q between irreducible representations of CW(w)C_W(w) and irreducible constituents of i0Hci(Xw,Qˉ)\bigoplus_{i\geq0}H^i_c(X_w,\bar{\mathbb Q}_\ell), and for each irreducible character χ\chi there is a polynomial Dχ(x)D_\chi(x), depending only on χ\chi, such that deg(χq)=Dχ(q)\deg(\chi_q)=D_\chi(q). The conjecture proposes a uniform description of the cohomology and its endomorphism algebra in terms of cyclotomic Hecke algebras; the supplied text does not state whether it has been resolved.

Sources & referencesView supporting material

Primary source

Andrew Mathas, “The representation theory of the Ariki-Koike and cyclotomic q-Schur algebras”, arXiv:math/0204025 (2002).

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