Problem books in higher mathematics Are there any problem book series on different topics with proper solutions? I have already found some in analysis (Problems in Mathematical Analysis 1-3) but not any in less popular topics.

I am very fond of the book 1001 Problems in Classical Number Theory by Jean-Marie De Koninck and Armel Mercier. As suggested by the title, the book consists of 1001 problems in number theory, broken up into the following sections: 


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*Mathematical Induction and Combinatorics 

*Divisibility, Prime Numbers

*Representation of Numbers

*Congruences 

*Primality Tests and Factorization Algorithms

*Integer Parts

*Arithmetical Functions

*Solving Equations Involving Arithmetical Functions

*Diophantine Equations

*Quadratic Reciprocity 

*Continues Fractions

*Classification of Real Numbers


The book has more than 200 pages of solutions, which often times provide a rough outline rather than a full solution, giving one a path to follow in order to fill in the details oneself. 
