Formal generalization of the π theorem for the case of arbitrarily many quantities was given first by A. Vaschy [ fr ] in 1892, then in 1911—apparently independently—by both A. Federman and D. Riabouchinsky, and again in 1914 by Buckingham. It was Buckingham's article that introduced the use of the symbol " π π i " for the dimensionless variables (or parameters), and this is the source of the theorem's name.