This text is missing some spaces, please add them: UsingmethodsofChasles,withSchröder'slogicalcalculusasamodel,hesetupasystemtosolvesuchproblems,hecalledittheprincipalofconservationofthenumber.Hilbert,in1900,askedforaproof,whichwasgivenbySeveriin1912.
Using methods of Chasles , with Schröder 's logical calculus as a model, he set up a system to solve such problems, he called it the principal of conservation of the number. Hilbert , in 1900, asked for a proof, which was given by Severi in 1912.