Elements of number theory / Anthony J. Pettofrezzo and Donald R. Byrkit. Pettofrezzo, Anthony J. Byrkit, Donald R. Number theory. Number theory Problems, exercises, etc. 512.81 PEE Includes index. Englewood, Cliffs, N. J. : Prentice-Hall, c.1970. c.1970. 1970 Text xii, 244 p. ; eng