Thanks for the reference, Ed!! And it contains a puzzle within a puzzle -- which 3 are duplicated, and are there solutions with some other 3 duplicated? For that matter, how many distinct ways can 3 of the double-6 set be duplicated? Meanwhile, my program found several distinct solutions to the smallest rectangular domino network with any solutions, a double-10 set (1..11, or blank..10) on a 4x10. There probably are many solutions. One is: 1 1 2 2 3 4 4 5 2 6 7 3 8 9 1 6 8 4 10 8 1 11 10 3 5 9 9 3 5 7 2 11 8 7 10 5 11 11 6 6 -- Mike ----- Original Message ----- From: "Ed Pegg Jr" <ed@mathpuzzle.com> To: "math-fun" <math-fun@mailman.xmission.com> Sent: Sunday, September 12, 2010 5:45 PM Subject: Re: [math-fun] domino networks I found an early reference. David Wells, Games and Puzzles, April 1976, page 31 Dominimum puzzle. He asks readers to fit the double-6 dominoes into a 4x5 grid. 3 dominoes are duplicated. --Ed Pegg Jr _______________________________________________ math-fun mailing list math-fun@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun