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A322764
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Number of set partitions of the multiset consisting of one copy each of x_1, x_2, ..., x_n, and 2 copies each of y_1 and y_2.
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5
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9, 26, 92, 371, 1663, 8155, 43263, 246218, 1493344, 9600683, 65133513, 464538351, 3471671717, 27109690422, 220646396816, 1867649896679, 16408260807503, 149357276866099, 1406334890073883, 13677748330883790, 137221985081833892
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OFFSET
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0,1
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COMMENTS
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The initial 9 is also A020555(2).
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REFERENCES
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D. E. Knuth, The Art of Computer Programming, Vol. 4A, Table A-1, page 778.
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LINKS
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Seiichi Manyama, Table of n, a(n) for n = 0..500
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FORMULA
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4*a(n) = 3*b(n) + 2*b(n+1) + 3*b(n+2) + 2*b(n+3) + b(n+4), where b(n) = A000110(n). - Seiichi Manyama, Nov 21 2020
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PROG
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(PARI) T(n, k) = if(k==0, sum(j=0, n, stirling(n, j, 2)), (T(n+2, k-1)+T(n+1, k-1)+sum(j=0, k-1, binomial(k-1, j)*T(n, j)))/2);
vector(20, n, T(n-1, 2)) \\ Seiichi Manyama, Nov 21 2020
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CROSSREFS
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Cf. A000110 (Bell number), A020555, A322773.
Column 2 of the array in A322765.
Sequence in context: A055849 A235163 A084813 * A325585 A056409 A056399
Adjacent sequences: A322761 A322762 A322763 * A322765 A322766 A322767
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KEYWORD
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nonn
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AUTHOR
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N. J. A. Sloane, Dec 30 2018
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STATUS
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approved
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