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 A227937 #20 Oct 09 2013 04:12:34
%S A227937 1,0,1,1,3,10,25,105,385,1540,7245,32725,164395,870870,4689685,
%T A227937 27152125,161786625,997196200,6443061625,42702885225,292938721075,
%U A227937 2078239413250,15119319039825,113390111659825,873538909468225,6894294734827500,55855506234653125,463151808682688125,3927996120260086875,34081631999814148750,301951521812713898125,2731127272307562253125,25208456056107710010625,237164027532948085570000
%N A227937 Partitions of n labeled elements into subsets of two or three elements.
%C A227937 Periodic modulo two and modulo three, but appears to eventually be divisible by other prime powers.
%F A227937 a(n) = (n-1)*a(n-2) + (n-1)*(n-2)*a(n-3)/2.
%F A227937 E.g.f.: exp( x^2/2 + x^3/6 ). [_Joerg Arndt_, Oct 07 2013]
%F A227937 a(n) ~ n^(2*n/3) * 2^(-n/3) * exp(2/9 - 2*n/3 - (2*n)^(1/3)/3 + (2*n)^(2/3)/2)/sqrt(3) * (1 + 34/(162*(2*n)^(1/3)) - 34802/(131220*(2*n)^(2/3))). - _Vaclav Kotesovec_, Oct 09 2013
%e A227937 The five elements a, b, c, d, e have ten partitions into sets of size two or three: ab/cde, ac/bde, ad/bce, ae/bcd, bc/ade, bd/ace, be/acd, cd/abe, ce/abd, and de/abc.
%t A227937 Flatten[{1,RecurrenceTable[{2*a[n] - 2*(n-1)*a[n-2]-(n-2)*(n-1)*a[n-3] == 0,a[1]==0,a[2]==1,a[3]==1}, a, {n, 1, 20}]}] (* _Vaclav Kotesovec_, Oct 09 2013 *)
%o A227937 (PARI) x='x+O('x^66); Vec( serlaplace( exp( x^2/2 + x^3/6 ) ) ) \\ _Joerg Arndt_, Oct 07 2013
%Y A227937 Cf. A000296, A123023.
%K A227937 nonn,easy
%O A227937 0,5
%A A227937 _David Eppstein_, Oct 06 2013