Linear Transformations and Reduction Formulas for the Gelfand Hypergeometric Functions Associated with the Grassmannians $G_{2,4}$ and $G_{3,6}$

We show that the Gelfand hypergeometric functions associated with the Grassmannians $G_{2,4}$ and $G_{3,6}$ with some special relations imposed on the parameters can be represented in terms of hypergeometric series of a simpler form. In particular, a function associated with the Grassmannian $G_{2,4}$ (the case of three variables) can be represented (depending on the form of the additional conditions on the parameters of the series) in terms of the Horn series $H_2,G_2$, of the Appell functions $F_1,F_2,F_3$ and of the Gauss functions $F^2_1$, while the functions associated with the Grassmannian $G_{3,6}$ (the case of four variables) can be represented in terms of the series $G_2,F_1,F_2,F_3$ and$F^2_1$. The relation between certain formulas and the Gelfand–Graev–Retakh reduction formula is discussed. Combined linear transformations and universal elementary reduction rules underlying the method were implemented by a computer program developed by the authors on the basis of the computer algebra system Maple V-4.