L-packet multiplicity and integral structure in the K-theory of real inner forms

Abstract

Let $G$ be a connected linear real semisimple group with finite centre and discrete series, and let $G_c$ be a fixed compact inner form. We isolate an integral structure behind the discrete-series character identity. There are natural homomorphisms ${\mathcal{C}}_G:K_0(C_r^{\ast }(G))⟶R(G_c)$ and ${\mathcal{J}}_G:R(G_c)⟶K_0(C_r^{\ast }(G))$, characterised, respectively, by stable and ordinary elliptic orbital integrals. The first sends a noncompact Dolbeault--Dirac index to its compact counterpart; the second sends an irreducible representation of $G_c$ to the signed sum of the $K$-theory classes in the corresponding discrete-series $L$-packet. We prove ${\mathcal{C}}_G{\mathcal{J}}_G=[W_G:W_K]{\mathrm{id}}_{R(G_c)}$. Thus the packet cardinality is the precise integral cost of splitting stable orbital averaging. Writing $S_G=\mathrm{im}{\mathcal{J}}_G$ and $U_G=\ker {\mathcal{C}}_G$, we obtain the exact obstruction sequence $0⟶S_G\oplus U_G⟶K_0(C_r^{\ast }(G))⟶R(G_c)/[W_G:W_K]R(G_c)⟶0$. After inverting the packet cardinality, this yields a canonical stable projector and functorial transfers between the stable $K$-theory lattices of real inner forms. For $\mathrm{SL}(2,\mathbb{R})$ the obstruction is $(\mathbb{Z}/2\mathbb{Z})[z+z^{-1}]$. For the inner forms of type $C_n$, the relevant multiplier is $2^n$ for $\mathrm{Sp}(2n,\mathbb{R})$ and $\left(\genfrac{}{}{0ex}{}{n}{p}\right)$ for $\mathrm{Sp}(p,n-p)$.