1. I. Herstein and I. Kaplansky, Matters Mathematical, Harper & Row, New York, 1974.
2. I. Niven and H. Zuckerman, An Introduction to the Theory of Numbers, 3rd ed., Wiley, New York, 1972.
3. H. Royden, Real Analysis, 2nd ed., Macmillan, New York, 1971.