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Number Theory
Algorithmic Number Theory Symposium
Fast Library for Number Theory
Lagrange's four-square theorem
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Number theory: an introduction to mathematics, W. A. Coppel
Number Theory: An Approach Through History from Hammurapi to Legendre, André Weil
Number theory: an introduction via the distribution of primes, Benjamin Fine, Gerhard Rosenberger
Fundamental number theory with applications, Richard A. Mollin
Classical theory of algebraic numbers, Paulo Ribenboim
Unsolved problems in number theory, Richard K. Guy
Old and new unsolved problems in plane geometry and number theory, Victor Klee, Stan. Wagon