Combinatorial

k-subsets

• ranking and unranking functions
• Lexicographic and colexicographic ordering

See

• Derrick Stolee, Ranking and Unranking of Combinations and Permutations, http://computationalcombinatorics.wordpress.com/2012/09/10/ranking-and-unranking-of-combinations-and-permutations/
• Wikipedia, Colexicographic order, http://en.wikipedia.org/wiki/Colexicographical_order#Colexicographic_order
  Also, OEIS, Orderings https://oeis.org/wiki/Orderings#Colexicographic_order.
• Gordon Royle, Combinatorial Enumeration: Theory and Practice, http://undergraduate.csse.uwa.edu.au/units/CITS7209/lecture02.pdf
• Mircea Marin, Combinations and k-Combinations: Ranking and Unranking Algorithmsn http://web.info.uvt.ro/~mmarin/lectures/GTC/AppendixL3.pdf
• Stephen G. Hartke, Ranking and Unranking of k-Subsets, http://www.math.ucsd.edu/~gptesler/184a/slides/rank_s13-handout.pdf