Shuffle product for multiple zeta functions

In this paper, we investigate the shuffle product relations for Euler-Zagier multiple zeta functions as functional relations. To this end, we generalize the classical partial fraction decomposition formula and give two proofs. One is based on a connection formula for Gauss's hypergeometric functions, the other one is based on an elementary calculus. Though it is hard to write down explicit formula of the shuffle product relations for multiple zeta functions as in the case of multiple zeta values, we will provide inductive steps by using the zeta-functions of root systems. As an application, we get the functional double shuffle relations for multiple zeta functions and show that some relations for multiple zeta values/functions can be deduced from our results.