Expanded endoscopic conjecture for equal weights at level 2

Let Vm,m\mathbb V_{m,m} be the equal-weight local system on A2[2]\mathcal A_2[2], set k=2m+4k=2m+4, and write τN,k=dimSk(Γ0(N))new\tau_{N,k}=\dim S_k(\Gamma_0(N))^{\rm new} and τk±=dimSk±(Γ0(2))new\tau_k^\pm=\dim S_k^\pm(\Gamma_0(2))^{\rm new}. Let s[λ]s[\lambda] denote the irreducible S6\mathbb S_6-representation indexed by λ\lambda. Equal-weight expanded endoscopic conjecture. The expanded endoscopic part is

Lm+1(L+1){τk+s[16]+τ4,ks[32]+τks[5,1],m odd,(τk+τ1,k)s[23]+(τk++τ1,k)s[4,2]+τ1,ks[6],m even.L^{m+1}(L+1)\begin{cases} \tau_k^+s[1^6]+\tau_{4,k}s[3^2]+\tau_k^-s[5,1],&m\text{ odd},\\ (\tau_k^-+\tau_{1,k})s[2^3]+(\tau_k^++\tau_{1,k})s[4,2]+\tau_{1,k}s[6],&m\text{ even}. \end{cases}

This is the equal-weight counterpart of the expanded endoscopic prediction and accounts for the parity-dependent Saito–Kurokawa contributions.

Sources & referencesView supporting material

Primary source

Jonas Bergström, Carel Faber and Gerard van der Geer, “Siegel modular forms of genus 2 and level 2: cohomological computations and conjectures”, arXiv:0803.0917 (2008).

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