“…[7, 25, 27, 37, 511 55, 56, 65, 66, 90, 97, 98, 106, 134, 136, 145]), expansions in orthogonal systems of functions and special functions (cf. [2,7,51,55,56,114,131,136,[142][143][144]183]), spectral decomposition of operators (cf. [3,5,51,55,56,135,149,150,178]), duality theory (cf.…”