The shrunken Weyl chamber conjecture for affine Deligne–Lusztig varieties

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Let GG be the reductive group in the setup, let x=wεμ∈W~x=w\varepsilon^\mu\in\widetilde W lie in a shrunken Weyl chamber, so that LP⁡(x)={v}\operatorname{LP}(x)=\{v\} for a uniquely determined v∈Wv\in W, and let [b]∈B(G)[b]\in B(G). If Xx(b)≠∅X_x(b)\neq\emptyset, define D∈Z≥0D\in\mathbb Z_{\geq 0} by

dim⁡Xx(b)=12(ℓ(x)+D−⟨ν(b),2ρ⟩−defect⁡(b)),\dim X_x(b)=\frac{1}{2}\left(\ell(x)+D-\langle\nu(b),2\rho\rangle-\operatorname{defect}(b)\right),

and let C∈Z≥1C\in\mathbb Z_{\geq 1} be the number of Jb(F)J_b(F)-orbits of top-dimensional irreducible components of Xx(b)X_x(b). Write λ(b)\lambda(b) for the relevant Kottwitz invariant and define either of the multisets

E1={e∣(ω,e)∈wts⁡(v⇒σ(wv)), λ(b)≡v−1μ−ω∈X∗(T)Γ}m,E_1=\{e\mid(\omega,e)\in\operatorname{wts}(v\Rightarrow\sigma(wv)),\ \lambda(b)\equiv v^{-1}\mu-\omega\in X_\ast(T)_\Gamma\}_m, E2={e∣(ω,e)∈wts⁡(v⇒σ(wv)), ν(b)=conv⁡(v−1μ−ω)}m.E_2=\{e\mid(\omega,e)\in\operatorname{wts}(v\Rightarrow\sigma(wv)),\ \nu(b)=\operatorname{conv}(v^{-1}\mu-\omega)\}_m.

Let EE be either E1E_1 or E2E_2. The shrunken Weyl chamber conjecture. The predictions are: (a) Xx(b)≠∅X_x(b)\neq\emptyset if and only if E≠∅E\neq\emptyset and κ(x)=κ(b)∈π1(G)Γ\kappa(x)=\kappa(b)\in\pi_1(G)_\Gamma; for E=E1E=E_1, the Kottwitz condition is automatic. (b) If Xx(b)≠∅X_x(b)\neq\emptyset, then max⁡(E)=D\max(E)=D. (c) If Xx(b)≠∅X_x(b)\neq\emptyset, then CC is at most the multiplicity of DD in EE, which may be +∞+\infty for E2E_2.

References

Primary source

Felix Schremmer, “Affine Deligne-Lusztig varieties via the double Bruhat graph II: Iwahori-Hecke algebra”, arXiv:2306.06873 (2024).

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