1. J. Delsartes, ?Sur certaines transformations fonctionelles relative aux equations lineaires aux derivees partielles du second ordre,? C. R. Acad. Sci. Paris,206, No. 1, 178?182 (1938).
2. J. Delsartes and J. L. Lions, ?Transmutations d'opèrateurs differentielles dans le domain complexe,? Comment. Math. Helv.,32, No. 2, 113?128 (1957).
3. L. A. Sakhnovich, ?Inverse problem for differential operators of order n>2 with analytic coefficients,? Mat. Sb.,46, No. 1, 61?76 (1958).
4. M. K. Fage, ?Operator analytic functions of one independent variable,? Dokl. Akad. Nauk SSSR,112, No. 6, 1008?1011 (1957).
5. M. K. Fage, ?Operator analytic functions of one independent variable,? Tr. Mosk. Mat. O-va, No. 7, 227?268 (1958).