Michael Ferris's blog
Permutations in GAMS
See: www.gams.com/testlib/libhtml/perm1.htm
Used in traveling salesman problem: www.gams.com/modlib/libhtml/ptsp.htm
Erwin blog entry: yetanothermathprogrammingconsultant.blogspot.com/2012_04_01_archive.html
Also note that information in ~/Research/permutation