§Number Theory: 1. Six proofs of the infinity of primes.- 2. Bertrand's postulate.- 3. Binomial coefficients are (almost) never powers.- 4. Representing numbers as sums of two squares.- 5. The law of quadratic reciprocity.- 6. Every finite division ring is a field.- 7. The spectral theorem and Hadamard's determinant problem.- 8. Some irrational numbers.- 9. Three times pi2/6.- Geometry: 10. Hilbert's third problem: decomposing polyhedral.- 11. Lines in the plane and decompositions of graphs.- 12. The slope problem.- 13. Three applications of Euler's formula.- 14. Cauchy's rigidity theorem.- 15. The Borromean rings don't exist.- 16. Touching simplices.- 17. Every large point set has an obtuse angle.- 18. Borsuk's conjecture.- Analysis: 19. Sets, functions, and the continuum hypothesis.- 20. In praise of inequalities.- 21. The fundamental theorem of algebra.- 22. One square and an odd number of triangles.- 23. A theorem of Pólya on polynomials.- 24. On a lemma of Littlewood and Offord.- 25. Cotangent and the Herglotz trick.- 26. Buffon's needle problem.- Combinatorics: 27. Pigeon-hole and double counting.- 28. Tiling rectangles.- 29. Three famous theorems on finite sets.- 30. Shuffling cards.- 31. Lattice paths and determinants.- 32. Cayley's formula for the number of trees.- 33. Identities versus bijections.- 34. The finite Kakeya problem.- 35. Completing Latin squares.- Graph Theory: 36. The Dinitz problem.- 37. Permanents and the po§wer of entropy.- 38. Five-coloring plane graphs.- 39. How to guard a museum.- 40. Turán's graph theorem.- 41. Communicating without errors.- 42. The chromatic number of Kneser graphs.- 43. Of friends and politicians.- 44. Probability makes counting (sometimes) easy.- About the Illustrations.- Index.§§