I am looking for the following paper by Gelfand & Ponomarev:
I. M. Gelfand and V. A. Ponomarev, Problems of linear algebra and classification of quadruples of subspaces in a finite-dimensional vector space, Colloquia Mathematica Societatis Ianos Bolyai, 5, Hilbert space operators, Tihany (Hungary), 1970,163—237
Many thanks in advance for finding the paper for me!