Problem 26

Prove that every composite number has a proper factor less than or equal to its square root.

Problem 21

Prove that the a composite n non divisible by   is a semiprime.

Problem 18

Given a Pythagorean triple a, b, c, prove that if a is odd, then either b or c must be odd.

Problem 16

Prove that there are an infinite number of Pythagorean triples.