EN
Let X be a closed subspace of $L^{p}(μ)$, where μ is an arbitrary measure and 1 < p < ∞. Let U be an invertible operator on X such that $sup_{n∈ ℤ} ||Uⁿ|| < ∞$. Motivated by applications in ergodic theory, we obtain (optimal) conditions for the convergence of series like $∑_{n≥1} (Uⁿf)/n^{1-α}$, 0 ≤ α < 1, in terms of $||f + ⋯ + U^{n-1}f||_{p}$, generalizing results for unitary (or normal) operators in L²(μ). The proofs make use of the spectral integration initiated by Berkson and Gillespie and, more particularly, of results from a paper by Berkson-Bourgain-Gillespie.