
Keywords: 41A25; 41A65; 41A99; 46B50; 46C05; 46C07; 46C99; 46N10; 46N99; 47B15; 52A07; 52A27; Hilbert spaces; Inversion of positive self-adjoint operators; Moore-Penrose generalized inverse; Inversion of frame operator; Strongly annihilating subspaces; Approximat