University of California, San Diego: Edward Bender and S. Williamson's "Arithmetic, Logic and Numbers: Number Theory"
Read over the Exercises for Section 1 on pages NT-10 - NT-13. Note the use of counterexamples. A universally quantified statement may be false. It can be proven false by showing that there exists an instance of the universally bound variable for which the statement is false. The instance is called a counter example. Exercises often have useful information, also applicable outside of the exercise.