  The method of dimensional recurrences proposed by one of the authors [1,2] is
applied to the evaluation of the pentagon-type scalar integral with on-shell
external legs and massless internal lines. For the first time, an analytic
result valid for arbitrary space-time dimension d and five arbitrary kinematic
variables is presented. An explicit expression in terms of the Appell
hypergeometric function F_3 and the Gauss hypergeometric function_2F_1, both
admitting one-fold integral representations, is given. In the case when one
kinematic variable vanishes, the integral reduces to a combination of Gauss
hypergeometric functions_2F_1. For the case when one scalar invariant is large
compared to the others, the asymptotic values of the integral in terms of Gauss
hypergeometric functions_2F_1 are presented in d = 2 - 2 epsilon, 4 - 2
epsilon, and 6 - 2 epsilon dimensions. For multi-Regge kinematics, the
asymptotic value of the integral in d = 4 - 2 epsilon dimensions is given in
terms of the Appell function F_3 and the Gauss hypergeometric function_2F_1.
