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 A306191 T(n,k) is a triangular array read by rows. Let S_n act on the set of size two subsets of {1,2,...,n}. T(n,k) is the number of permutations in S_n that fix exactly k size two subsets, n >= 1, 0 <= k <= binomial(n,2). 0

%I #11 Feb 17 2019 09:29:22

%S 1,0,2,2,3,0,1,14,0,9,0,0,0,1,54,40,15,0,10,0,0,0,0,0,1,304,300,0,100,

%T 0,0,0,15,0,0,0,0,0,0,0,1,2260,1638,630,315,0,105,70,0,0,0,0,21,0,0,0,

%U 0,0,0,0,0,0,1,18108,12992,5460,1344,1645,0,420,0,210,0,112,0,0,0,0,0,28,0,0,0,0,0,0,0,0,0,0,0,1

%N T(n,k) is a triangular array read by rows. Let S_n act on the set of size two subsets of {1,2,...,n}. T(n,k) is the number of permutations in S_n that fix exactly k size two subsets, n >= 1, 0 <= k <= binomial(n,2).

%C The action of S_n on the 2-subsets of {1,2,...,n} is defined: For all pi in S_n, pi({i,j}) = {pi(i),pi(j)}.

%e 1,

%e 0, 2,

%e 2, 3, 0, 1,

%e 14, 0, 9, 0, 0, 0, 1,

%e 54, 40, 15, 0, 10, 0, 0, 0, 0, 0, 1,

%e 304, 300, 0, 100, 0, 0, 0, 15, 0, 0, 0, 0, 0, 0, 0, 1,

%t f[list_] := Flatten[Position[list /. x_ /; x > 0 -> 1, 1]];

%t Level[CoefficientList[Table[n! PairGroupIndex[SymmetricGroup[n], s] /. {Table[s[i] -> 1, {i, 2, Binomial[n, 2]}]}, {n, 1, 8}],

%t s[1]], {2}] // Grid

%Y Cf. A137482 is column 1.

%K nonn,tabf

%O 1,3

%A _Geoffrey Critzer_, Jan 28 2019

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