• Extraordinarily elegant design.

    http://norvig.com/sudoku.html

  • Another complete solution:

    http://www2.warwick.ac.uk/fac/sci/moac/people/students/peter_cock/python/sudoku

  • How not to ask a question on stackoverflow

    http://stackoverflow.com/questions/19553418/python-recursive-backtracking-suduko-solver

  • I don't understand this and I'm not sure I believe it.

    https://freepythontips.wordpress.com/2013/09/01/sudoku-solver-in-python/