QUADRATIC UNIPOTENT ARTHUR PARAMETERS AND RESIDUAL SPECTRUM OF SYMPLECTIC GROUPS

The purpose of this paper is to study certain quadratic unipotent Arth ur parameters in the sense of Moeglin and use them to parametrize a pa rt of the residual spectrum of symplectic groups over number fields, c oming from the conjugacy class of Borel subgroups. In particular, usin g certain identities satisfied by local intertwining operators, Arthur 's multiplicity formula is established for them which remarkably enoug h appears by itself in the corresponding residue of the Eisenstein ser ies.