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,...http://books.google.com/books/about/The_Ultimate_Challenge.html?id=hekJ7JDMEVkC&utm_source=gb-gplus-shareThe Ultimate Challenge