Dudas–Malle projectivity conjecture for intersection cohomology characters

Let GG be a finite reductive group with Weyl group WW, let FF be the relevant Frobenius endomorphism, and let T\boldsymbol{T} be an FF-stable torus. For wotinWw otin W, let QwQ_w denote the relevant character, let DGD_G denote Alvis–Curtis duality, and let X(w)\overline{\boldsymbol{X}(w)} be the closure of the Deligne–Lusztig variety. Write IH(X(w),k(TwF))IH^\bullet(\overline{\boldsymbol{X}(w)},k(\boldsymbol{T}^{wF})_\ell) for its intersection cohomology with the indicated coefficient module. Dudas–Malle conjecture. For all wWw\in W, there is a sign εw=±1\varepsilon_w=\pm1 such that

Qw=εwDG([IH(X(w),k(TwF))])Q_w=\varepsilon_w D_G\bigl([IH^\bullet(\overline{\boldsymbol{X}(w)},k(\boldsymbol{T}^{wF})_\ell)]\bigr)

is the character of a non-virtual projective module. This conjecture proposes a geometric construction of projective modules from intersection cohomology and would provide the projectives needed to control modular decomposition matrices. The source presents it as a proposed strategy, without reporting a proof or disproof.

Sources & referencesView supporting material

Primary source

Olivier Dudas, “Lectures on modular Deligne–Lusztig theory”, arXiv:1705.08234 (2017).

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