This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A006541 M4920 #34 Jul 02 2025 16:01:55
%S A006541 1,1,13,4683,102247563,230283190977853,81124824998504073881821,
%T A006541 6297562064950066033518373935334635,
%U A006541 144199280951655469628360978109406917583513090155,1255482482235481041484313695469155949742941807533901307975355741
%N A006541 Number of dissimilarity relations on an n-set.
%D A006541 M. Schader, Hierarchical analysis: classification with ordinal object dissimilarities, Metrika, 27 (1980), 127-132.
%D A006541 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A006541 Vincenzo Librandi, <a href="/A006541/b006541.txt">Table of n, a(n) for n = 1..29</a>
%H A006541 M. Schader, <a href="/A005121/a005121_1.pdf">Hierarchical analysis: classification with ordinal object dissimilarities</a>, Metrika, 27 (1980), 127-132. [Annotated scanned copy]
%H A006541 M. Schader, <a href="/A005121/a005121.pdf">Letter to N. J. A. Sloane</a>, Aug 25 1981.
%F A006541 a(n) = Sum_{i=0..m} (m-i)!*Stirling2(m, m-i), where m = n*(n-1)/2.
%F A006541 a(n) = A000670(n*(n-1)/2).
%p A006541 b:= proc(n, k) b(n, k):= `if`(n=0, k!, k*b(n-1, k)+b(n-1, k+1)) end:
%p A006541 a:= n-> b(n*(n-1)/2, 0):
%p A006541 seq(a(n), n=1..12); # _Alois P. Heinz_, Dec 02 2024
%t A006541 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_ *)
%o A006541 (PARI) a(n)=ceil(polylog(-n*(n-1)/2, 1/2)/2) \\ _Charles R Greathouse IV_, Aug 27 2014
%Y A006541 Cf. A000670, A005121.
%K A006541 nonn,nice,easy
%O A006541 1,3
%A A006541 _N. J. A. Sloane_
%E A006541 More terms from _James Sellers_, Jan 19 2000