A014489 Positions of involutions (permutations whose square is the identity) in reverse colexicographic order (A055089/A195663).

0, 1, 2, 5, 6, 7, 14, 16, 21, 23, 24, 25, 26, 29, 54, 55, 60, 67, 80, 82, 86, 94, 105, 107, 111, 119, 120, 121, 122, 125, 126, 127, 134, 136, 141, 143, 264, 265, 266, 269, 288, 289, 314, 316, 339, 341, 390, 391, 396, 403, 414, 415, 444, 450, 469

Offset: 0

Author

Wouter Meeussen

Keywords

• nonn

Links

• Robert Israel, Table of n, a(n) for n = 0..3996

Crossrefs

Positions of zeros in A261099.

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

Subsequences: A060112, A064640.

Cf. A055089, A195663.

Cf. also A261220.

Programs

Maple

N:= 100: # to get a(0) to a(N)
M:= 0: A[0]:= 0: count:= 0:
for m from 2 while count < N do
  P:= remove(t -> t[1]=1, combinat:-permute(m));
  P:= map(t -> ListTools:-Reverse(subs([seq(i=m+1-i,i=1..m)],t)),P);
  R:= select(t -> max(map(nops,convert(P[t],disjcyc))) = 2, [$1..nops(P)]);
  for r in R do
     count:= count+1;
     A[count]:= r+M;
     if count = N then break fi;
  od:
  M:= M + nops(P);
od:
seq(A[i],i=0..count); # Robert Israel, Oct 28 2015

Extensions

Name changed by Antti Karttunen, Aug 30 2015