1. What Is Number Theory? 2. Pythagorean Triples 3. Pythagorean Triples and the Unit Circle 4. Sums of Higher Powers and Fermat’s Last Theorem 5. Divisibility and the Greatest Common Divisor 6. Linear Equations and the Greatest Common Divisor 7. Factorization and the Fundamental Theorem of Arithmetic 8. Congruences 9. Congruences, Powers, and Fermat’s Little Theorem 10. Congruences, Powers, and Euler’s Formula 11. Euler’s Phi Function and the Chinese Remainder Theorem 12. Prime Numbers 13. Counting Primes 14. Mersenne Primes 15. Mersenne Primes and Perfect Numbers8 16. Powers Modulo m and Successive Squaring 17. Computing kth Roots Modulo m 18. Powers, Roots, and “Unbreakable” Codes 19. Primality Testing and Carmichael Numbers 20. Euler’s Phi Function and Sums of Divisors 21. Powers Modulo p and Primitive Roots 22. Primitive Roots and Indices 23. Squares Modulo p 24. Is —1 a Square Modulo p? Is 2? 25. Quadratic Reciprocity 26. Which Primes Are Sums of Two Squares? 27. Which Numbers Are Sums of Two Squares? 28. The Equation X4 + Y 4 = Z4 29. Square-Triangular Numbers Revisited 30. Pell’s Equation 31. Diophantine Approximation 32. Diophantine Approximation and Pell’s Equation 33. Number Theory and Imaginary Numbers 34. The Gaussian Integers and Unique Factorization 35. Irrational Numbers and Transcendental Numbers 36. Binomial Coefficients and Pascal’s Triangle 37. Fibonacci’s Rabbits and Linear Recurrence Sequences 38. Oh, What a Beautiful Function 39. The Topsy-Turvy World of Continued Fractions 40. Continued Fractions, Square Roots and Pell’s Equation 41. Generating Functions 42. Sums of Powers 43. Cubic Curves and Elliptic Curves 44. Elliptic Curves with Few Rational Points 45. Points on Elliptic Curves Modulo p 46. Torsion Collections Modulo p and Bad Primes 47. Defect Bounds and Modularity Patterns 48. Elliptic Curves and Fermat’s Last Theorem Further Reading A. Factorization of Small Composite Integers B. A List of Primes Index