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A014489
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Positions of involutions (permutations whose square is the identity) in reverse colexicographic order (A055089/A195663).
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3
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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
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OFFSET
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0,3
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LINKS
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MAPLE
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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:
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CROSSREFS
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From a(1)=1 onward also positions of 2's in A055092.
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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