In a Hilbert space, for orthorecursive expansions with respect to closed subspaces, we establish a criterion for expansions of elements of a certain finite-dimensional subspace with respect to a finite sequence of subspaces to coincide with the expanded elements. This implies a criterion for an element to be equal to its orthorecursive expansion with respect to a finite sequence of subspaces. We also obtain a number of results related to the best approximations of elements by partial sums of their orthorecursive expansions with respect to a sequence of finite-dimensional subspaces.
机构:
St Petersburg State Univ, Dept Math, St Petersburg 198504, Petrodvorets, RussiaSt Petersburg State Univ, Dept Math, St Petersburg 198504, Petrodvorets, Russia