Spectral multiplicity of the solutions of polynomial operator equations

In the note one considers operators T, acting in a Hilbert space and satisfying an equation of the form (T)=A, where is a polynomial, while A is a given normal operator, assumed to be either reductive or unitary. Under these conditions one computes some spectral characteristics of the operator T (spectral multiplicity, disc, lattice of invariant subspaces, etc.). Fundamental examples are the weighted substitution operators (TL²(X,)L²(X,), Tf=·(f·), where is a periodic automorphism of (X,), L (X, ).Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 157, pp. 157–164, 1987.The author expresses his sincere gratitude to N. K. Nikol'skii for the formulation of the problem and for the useful discussion of the results.