Quadratic unipotent Arthur parameters and residual spectrum of symplectic groups

The purpose of this paper is to study certain quadratic unipotent Arthur parameters in the sense of Moeglin and use them to parametrize a part of the residual spectrum of symplectic groups over number fields, coming from the conjugacy class of Borel subgroups. In particular, using certain identities satisfied by local intertwining operators, Arthur's multiplicity formula is established for them which remarkably enough appears by itself in the corresponding residue of the Eisenstein series.