Biorthogonal Appell-like systems in a Hilbert space

Kachanovsky, N. A. “Biorthogonal Appell-Like Systems in a Hilbert Space”. Methods of Functional Analysis and Topology, vol. 2, no. 3, Dec. 1996, pp. 36-52, https://zen.imath.kiev.ua/index.php/mfat/article/view/18.