"With a view to recalling Clausen's identity, we begin by introducing the generalized Gaussian and Clausenian hypergeometric function defined, in the notations of Leo Pochhammer and Ernest William Barnes... as already pointed out by Barnes, the generalized hypergeometric function pFq originated with Clausen and was studied, among others, by Johannes Karl Thomae, Édouard Jean-Baptiste Goursat, and Pochhammer whose voluminous work on the subject provides a detailed development of the theory."