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On the $η$-function for bisingular pseudodifferential operators

In this work we consider the $η$-invariant for pseudodifferential operators of tensor product type, also called bisingular pseudodifferential operators. We study complex powers of classical bisingular operators. We prove the trace property for the Wodzicki residue of bisingular operators and show how the residues of the $η$-function can be expressed in terms of the Wodzicki trace of a projection operator. Then we calculate the $K$-theory of the algebra of $0$-order (global) bisingular operators. With these preparations we establish the regularity properties of the $η$-function at the origin for global bisingular operators which are self-adjoint, elliptic and of positive orders.