1. Congruence

2. $\Z_n$

3. The Euclidean Algorithm

4. $\U_n$

5. The Fundamental Theorem of Arithmetic

6. The GCD and the LCM

7. The Chinese Remainder Theorem

8. The Euler Phi Function

9. The Phi Function—Continued

10. Wilson's Theorem and Euler's Theorem

11. Public Key Cryptography

12. Quadratic Reciprocity