Geometrization of the two orthogonality formulas for Green functions

Abstract

The Green functions were first introduced by Green to compute the character table of GLn(q) in 1955. They were later generalized by Deligne and Lusztig for an arbitrary finite group of Lie type G(q) using l-adic cohomological methods (1976). They proved that these Green functions satisfy an orthogonality relation (we call the first orthogonality relation). Ten years later Kawanaka proved that they satisfy an other orthogonality relation (we call the second orthogonality relation). In this notes, we provide a geometrical understanding of these two orthogonality relations and explain how we can see geometrically that the two orthogonality relations are in fact equivalent.