K(n)-local duality for finite groups and groupoids

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We define an inner product (suitably interpreted) on the K(n)-local spectrum LG:= Lₖ₍ₙ₎BG+, where G is a finite group or groupoid. This gives an inner product on E*BG+ for suitable K(n)-local ring spectra E. We relate this to the usual inner product on the representation ring when n=1, and to the Hopkins-Kuhn-Ravenel generalised character theory. We show that LG is a Frobenius algebra object in the K(n)-local stable category, and we recall the connection between Frobenius algebras and topological quantum field theories to help analyse this structure. In many places we find it convenient to use groupoids rather than groups, and to assist with this we include a detailed treatment of the homotopy theory of groupoids. We also explain some striking formal similarities between our duality and Atiyah-Poincare duality for manifolds.