2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/224788New proofs of the duplication formulae for the gamma and the Barnes double gamma functions are derived using the Hurwitz zeta function. Concise derivations of Gauss's multiplication theorem for the gamma function and a corresponding one for the double gamma function are also reported. This paper also refers to some connections with the Stieltjes constants.Classical Analysis and ODEs11M06text