A014489 - OEIS

%I #16 Nov 07 2015 08:33:46

%S 0,1,2,5,6,7,14,16,21,23,24,25,26,29,54,55,60,67,80,82,86,94,105,107,

%T 111,119,120,121,122,125,126,127,134,136,141,143,264,265,266,269,288,

%U 289,314,316,339,341,390,391,396,403,414,415,444,450,469

%N Positions of involutions (permutations whose square is the identity) in reverse colexicographic order (A055089/A195663).

%H Robert Israel, <a href="/A014489/b014489.txt">Table of n, a(n) for n = 0..3996</a>

%p N:= 100: # to get a(0) to a(N)

%p M:= 0: A[0]:= 0: count:= 0:

%p for m from 2 while count < N do

%p   P:= remove(t -> t[1]=1, combinat:-permute(m));

%p   P:= map(t -> ListTools:-Reverse(subs([seq(i=m+1-i,i=1..m)],t)),P);

%p   R:= select(t -> max(map(nops,convert(P[t],disjcyc))) = 2, [$1..nops(P)]);

%p   for r in R do

%p      count:= count+1;

%p      A[count]:= r+M;

%p      if count = N then break fi;

%p   od:

%p   M:= M + nops(P);

%p od:

%p seq(A[i],i=0..count); # _Robert Israel_, Oct 28 2015

%Y Positions of zeros in A261099.

%Y From a(1)=1 onward also positions of 2's in A055092.

%Y Subsequences: A060112, A064640.

%Y Cf. A055089, A195663.

%Y Cf. also A261220.

%K nonn

%O 0,3

%A _Wouter Meeussen_

%E Name changed by _Antti Karttunen_, Aug 30 2015