1. R. Courant and H. Robbins, What is Mathematics? Oxford Univ. Press, 1941.
2. W. J. LeVeque, Topics in Number Theory, vol. 1, Addison-Wesley, 1956.
3. Waclaw Sierpinski, Pythagorean Triangles. The Scripta Mathematica Studies, no. 9, Yeshiva Univ., New York, 1962.