New proofs of the duplication and multiplication formulae for the gamma and the Barnes double gamma functions

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New 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.