Topology of Numbers
by Allen Hatcher
Publisher: Cornell University 2014
Number of pages: 122
An introductory textbook on elementary number theory from a geometric point of view, as opposed to the usual strictly algebraic approach. A fair amount of the book is devoted to studying Conway's topographs associated to quadratic forms in two variables.
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by Thomas Taylor, A. J. Valpy
The substance of all that has been written on this subject by Nicomachus, Iamblichus, and Boetius, together with some particulars respecting perfect, amicable, and other numbers, which are not to be found in the writings of modern mathematicians.
by Wissam Raji - The Saylor Foundation
These are notes for an undergraduate course in number theory. Proofs of basic theorems are presented in an interesting and comprehensive way that can be read and understood even by non-majors. The exercises broaden the understanding of the concepts.
by Waclaw Sierpinski - ICM
The variety of topics covered here includes divisibility, diophantine equations, prime numbers, the basic arithmetic functions, congruences, the quadratic reciprocity law, expansion of real numbers into decimal fractions, and more.
by Leo Moser - The Trillia Group
The book on elementary number theory: compositions and partitions, arithmetic functions, distribution of primes, irrational numbers, congruences, Diophantine equations; combinatorial number theory, and geometry of numbers.