The OEIS is supported by the many generous donors to the OEIS Foundation.

 Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 59th year, we have over 358,000 sequences, and we’ve crossed 10,300 citations (which often say “discovered thanks to the OEIS”). Other ways to Give
 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A001691 Number of two-element generating sets in the symmetric group S_n. (Formerly M4660 N1995) 3
 0, 1, 9, 108, 3420, 114480, 7786800, 497266560, 42616445760, 4320959126400, 534444478444800, 77699101730342400, 13282131639801024000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 REFERENCES N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS J. Denes, Some combinatorial properties of transformations and their connections with the theory of graphs, J. Combin. Theory, 9 (1970), 108-116. FORMULA a(n) = A071605(n)/2 for n > 2. PROG (GAP) a := function(n) local tom, mu, lens, orders, num, k; tom := TableOfMarks(Concatenation("S", String(n))); if tom = fail then tom := TableOfMarks(SymmetricGroup(n)); fi; mu := MoebiusTom(tom).mu; lens := LengthsTom(tom); orders := OrdersTom(tom); num := 0; for k in [1 .. Length(lens)] do if IsBound(mu[k]) then num := num + mu[k] * lens[k] * Binomial(orders[k], 2); fi; od; return num; end; # Stephen A. Silver, Feb 20 2013 CROSSREFS Cf. A071605, A086373. Sequence in context: A166907 A051606 A330876 * A157906 A166846 A058421 Adjacent sequences: A001688 A001689 A001690 * A001692 A001693 A001694 KEYWORD nonn,more AUTHOR EXTENSIONS a(8)-a(9) (derived from A071605) added by Stephen A. Silver, Feb 17 2013 a(10)-a(13) added by Stephen A. Silver, Feb 20 2013 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 5 15:27 EST 2022. Contains 358588 sequences. (Running on oeis4.)