ON SOME EXTENSIONS OF THE A-MODEL.

The A-model for finite rank singular perturbations of class H-m-2∖H-m-1, m∈N, is considered from the perspective of boundary relations. Assuming further that the Hilbert spaces (Hn)n∈Z admit an orthogonal decomposition H-n⊕H+n, with the corresponding projections satisfying P±n+1⊆P±n, nontrivial extensions in the A-model are constructed for the symmetric restrictions in the subspaces. [ABSTRACT FROM AUTHOR]