A generalization of the Laurent series

A system of functions, biorthogonal to the D-basis introduced by M. K. Fage for a linear differential operator D with analytic coefficients, is constructed. Series in the combination of the D-basis with this system generalize Laurent series in the same way as series in the D-basis generalize Taylor series.Translated from Matematicheskie Zametki, Vol. 9, No. 5, pp. 521–531, May, 1971.