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 A006541 Number of dissimilarity relations on an n-set. (Formerly M4920) 2
 1, 1, 13, 4683, 102247563, 230283190977853, 81124824998504073881821, 6297562064950066033518373935334635, 144199280951655469628360978109406917583513090155 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 REFERENCES M. Schader, Hierarchical analysis: classification with ordinal object dissimilarities, Metrika, 27 (1980), 127-132. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..29 M. Schader, Hierarchical analysis: classification with ordinal object dissimilarities, Metrika, 27 (1980), 127-132. [Annotated scanned copy] M. Schader, Letter to N. J. A. Sloane, Aug 25 1981. FORMULA Sum_{i=0..m-1} (m-i)!*Stirling2(m, m-i), where m = n(n-1)/2. MATHEMATICA a[n_] := PolyLog[-n(n-1)/2, 1/2]/2; a[1]=1; Table[a[n], {n, 1, 9}] (* Jean-François Alcover, Jun 28 2012, after Wouter Meeussen *) PROG (PARI) a(n)=ceil(polylog(-n*(n-1)/2, 1/2)/2) \\ Charles R Greathouse IV, Aug 27 2014 CROSSREFS Cf. A005121. A000670(n(n-1)/2). Sequence in context: A203676 A023345 A249941 * A189647 A185674 A103857 Adjacent sequences:  A006538 A006539 A006540 * A006542 A006543 A006544 KEYWORD nonn,nice,easy AUTHOR EXTENSIONS More terms from James A. Sellers, Jan 19 2000 STATUS approved

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Last modified August 12 17:17 EDT 2020. Contains 336439 sequences. (Running on oeis4.)