The USSR Olympiad Problem Book: Selected Problems and Theorems of Elementary Mathematics:

Over 300 challenging problems in algebra, arithmetic, elementary number theory and trigonometry, selected from the archives of the Mathematical Olympiads held at Moscow University. Most presuppose only high school mathematics but some are of uncommon difficulty and will challenge any mathematician. Complete solutions to all problems. 27 black-and-white illustrations. 1962 edition.

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From Erdös to Kiev: Problems of Olympiad Caliber (Dolciani Mathematical Expositions):

Ross Honsberger’s love of mathematics comes through very clearly in From Erdös to Kiev. He presents intriguing, stimulating problems that can be solved with elementary mathematical techniques. It will give pleasure to motivated students and their teachers, but it will also appeal to anyone who enjoys a mathematical challenge. Most of the problems in the collection have appeared on national or international Olympiads or other contests. Thus, they are quite challenging (with solutions that are all the more rewarding). The solutions use straightforward arguments from elementary mathematics (often not very technical arguments) with only the occasional foray into sophisticated or advanced ideas. Anyone familiar with elementary mathematics can appreciate a large part of the book. The problems included in this collection are taken from geometry, number theory, probability, and combinatorics. Solutions to the problems are included.

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