books.google.com - The 3x+1 problem, or Collatz problem, concerns the following seemingly innocent arithmetic procedure applied to integers: If an integer x is odd then "multiply by three and add one", while if it is even then "divide by two." The 3x+1 problem asks whether, starting from any positive integer, repeating...http://books.google.com/books/about/The_Ultimate_Challenge.html?id=hekJ7JDMEVkC&utm_source=gb-gplus-shareThe Ultimate Challenge