Examples of books giving problems that require more than one branch of mathematics I want to know if there are books that give problem sets requiring knowledge of two or more branches of mathematics. For example, there could be a problem requiring geometry, set theory, and number theory, and another problem requiring ring theory and calculus. All the problems should be solvable with merely an undergraduate background.

Vector Calculus, Linear Algebra and Differential Forms: A Unified Approach by John and Barbara Hubbard offer many problems that tie together analysis and linear algebra. The problem sets however do not require as diverse of a knowledge as competition level problems (as stated by Alfred).
