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 A064638 #5 May 01 2014 02:49:07 %S A064638 0,1,7,23,127,415,143,659,719,5167,16687,5455,26815,28495,5183,16703, %T A064638 5699,36899,38579,5759,36959,40031,40319,368047,1174447,379567, %U A064638 1901647,1992367,368335,1174735,389695,2627455,2718175,391375,2629135,2799055 %N A064638 Positions of non-crossing fixed-point-free involutions encoded by A014486 in A055089. Permutation of A064640. %p A064638 map(PermRevLexRank,map(NonCrossingTranspos, A014486)); %p A064638 NonCrossingTranspos := n -> convert(NonCrossingTransposAux(binexp2pars(n),1),'permlist',binwidth(n)); %p A064638 NonCrossingTransposAux := proc(s,ii) local e,p,i,j; i := ii; p := []; for e in s do p := permul(p,NonCrossingTransposAux(e,i+1)); j := i+CountParens(e)+1; p := permul(p,[[i,j]]); i := j+1; od; RETURN(p); end; %p A064638 CountParens := proc(s) local e,k; if(0 = nops(s)) then RETURN(0); fi; e := 0; for k in s do e := e+2+CountParens(k); od; RETURN(e); end; %Y A064638 Maple procedure binexp2pars given in A057501, permul in A060125. %K A064638 nonn %O A064638 0,3 %A A064638 _Antti Karttunen_, Oct 02 2001