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A152525
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a(n) is the number of unordered pairs of disjoint set partitions of an n-element set.
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2
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0, 0, 1, 7, 65, 811, 12762, 244588, 5574956, 148332645, 4538695461, 157768581675, 6167103354744, 268758895112072, 12961171404183498, 687270616305277589, 39843719438374998543, 2512873126513271758171, 171643113190082528007702, 12647168303374365311984284
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OFFSET
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0,4
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LINKS
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FORMULA
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a(n) = Sum_{k=0..n} binomial(n,k) * (Sum_{j=0..n-k} (-1)^j*A048993(n-k,j)) * binomial(A000110(k),2).
That is, summed on k from 0 to n, the number of k-element subsets of an n-element set, times the alternating sum of row n-k of Stirling2 numbers starting with +S(n-k, 0), times the number of pairs of partitions of k elements.
Obtained by inverting (binomial(A000110(n), 2)) = (Sum_{k=0..n} binomial(n,k)*A000110(n-k)*a(k)), which in turn is gotten by considering that a pair of conjoint partitions is gotten by choosing a partition of a subset and then choosing a pair of disjoint partitions of the complement.
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EXAMPLE
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The a(3) = 7 unordered pairs:
{{1},{2},{3}}| {{1,2,3}}
{{1},{2,3}} |{{1,2},{3}}
{{1},{2,3}} |{{1,3},{2}}
{{1,2},{3}} |{{1,3},{2}}
{{1},{2,3}} | {{1,2,3}}
{{1,2},{3}} | {{1,2,3}}
{{1,3},{2}} | {{1,2,3}}
(End)
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MAPLE
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a:= n-> add(binomial(n, k)*binomial(combinat[bell](k), 2)*
add(Stirling2(n-k, j)*(-1)^j, j=0..n-k), k=0..n):
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MATHEMATICA
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Array[Sum[Binomial[#, k] Sum[(-1)^j*StirlingS2[# - k, j], {j, 0, # - k}] Binomial[BellB@ k, 2], {k, 0, #}] &, 20, 0] (* Michael De Vlieger, May 27 2018 *)
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PROG
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(PARI) a000110(n) = polcoeff( sum( k=0, n, prod( i=1, k, x / (1 - i*x)), x^n * O(x)), n);
a(n) = sum(k=0, n, binomial(n, k) * sum(j=0, n-k, (-1)^j*stirling(n-k, j, 2) * binomial(a000110(k), 2))); \\ Michel Marcus, May 27 2018
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CROSSREFS
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Cf. A000110, A000258, A001247, A008277, A048993, A059849, A060639, A181939, A193297, A318393, A322441, A322442, A320768.
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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