Sources of Elementary Number Theory Problems

Question

I am looking for sources of interesting and challenging problems that would suitably accompany an honors level introductory number theory course.

What are some good sources for interesting elementary number theory problems?

Edit: I am specifically looking for problems that would nicely complement a book such as Ivan Niven's Introduction to the Theory of Numbers that are not standard exercises, but rather interesting and challenging gems.

Answer 1

I totally recommend 250 problems of elementary number theory

Answer 2

The following seems to fit the bill, though I must admit to only taking a glance through.

http://www.math.muni.cz/~bulik/vyuka/pen-20070711.pdf