For a bit of an explanation, see David's follow-up blog post: http://blog.tanyakhovanova.com/2010/08/divisibility-by-7-is-a-walk-on-a-graph-ii-2/ http://blog.ta

For a bit of an explanation, see David's follow-up blog post: http://blog.tanyakhovanova.com/2010/08/divisibility-by-7-is-a-walk-on-a-graph-ii-2/ http://blog.tanyakhovanova.com/2010/08/divisibility-by-7-is-...