Embry truncated complex moment problem

Abstract

Let T be a cyclic subnormal operator on a Hilbert space ℋ with cyclic vector x0 and let γij:=(T*iT jx0,x0), for any i,j ∈ ℕ ∪ {0}. The Bram-Halmos' characterization for subnormality of T involved a moment matrix M(n). In a parallel approach, we construct a moment matrix E(n) corresponding to Embry's characterization for subnormality of T. We discuss the relationship between M(n) and E(n) via the full moment problem. Next, given a collection of complex numbers γ≡{γij} (0 ≤ i + j ≤ 2n, |i-j| ≤ n) with γ00 > 0 and γ ji = γ̄ij, we consider the truncated complex moment problem for γ; this entails finding a positive Borel measure μ supported in the complex plane ℂ such that γij = ∫z̄izjdμ(z). We show that this moment problem can be solved when E(n) ≥ 0 and E(n) admits a flat extension E(n + k), where k = 1 when n is odd and k = 2 when n is even. © 2003 Elsevier Inc. All rights reserved.