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A058681
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Number of matroids of rank 2 on n labeled points.
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28
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0, 0, 1, 7, 36, 171, 813, 4012, 20891, 115463, 677546, 4211549, 27640341, 190891130, 1382942161, 10480109379, 82864804268, 682076675087, 5832741942913, 51724157711084, 474869815108175, 4506715736350171, 44152005850890042
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OFFSET
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0,4
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COMMENTS
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Number of partitions of {1, 2, ..., n+1} in which at least one block of each partition contains a pair of nonconsecutive integers. E.g., B(4)-2^3 = 7: there are 7 partitions of {1,2,3,4} in which some block contains a pair of nonconsecutive integers, namely 124/3, 134/2, 14/23, 13/24, 13/2/4, 14/2/3, 1/24/3. - Augustine O. Munagi, Mar 20 2005
Number of complementing systems of subsets of {0, 1, ..., p^(n+1) - 1} (p a prime) in which at least one member is not of the form {0, x, 2x, ..., (c-1)x} for positive integers x and c. E.g., B(4)-p^3 = 7: there are 7 complementing systems of subsets of {0, 1, ..., p^4-1} in which at least one member is not of the form {0, x, 2x, ..., (c-1)*x}. Number of complementing systems of subsets of {0, 1, ..., p^4 - 1} reduces to B(4) and number of ordered factorizations of p^4 is p^3. - Augustine O. Munagi, Mar 20 2005
a(n) is the number of collections containing two or more nonempty subsets of {1,2,...,n} that are pairwise disjoint. - Geoffrey Critzer, Oct 10 2009
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LINKS
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FORMULA
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a(n) = B(n+1)-2^n, B = Bell numbers (A000110).
E.g.f.: d/dz (exp(exp(z)-1) - (1/2)*exp(2*z) - 1/2). - Thomas Wieder, Nov 30 2004
E.g.f.: exp(x + exp(x) - 1) - exp(2*x). - Peter Luschny, Jan 08 2021
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EXAMPLE
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a(3) = 7 because there are 7 collections (having more than one element)of nonempty subsets of {1,2,3} that are pairwise disjoint: {1}{2}; {1}{3}; {1}{2,3}; {2}{3}; {2}{1,3}; {1,2}{3}; {1}{2}{3}. - Geoffrey Critzer, Oct 10 2009
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MAPLE
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egf := exp(x + exp(x) - 1) - exp(2*x); ser := series(egf, x, 24):
seq(simplify(n!*coeff(ser, x, n)), n=0..22); # Peter Luschny, Jan 08 2021
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MATHEMATICA
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f[n_] := Sum[ StirlingS2[n + 1, k+2], {k, 1, n}]; Table[ f[n], {n, 0, 23}] - Zerinvary Lajos, Mar 21 2007
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CROSSREFS
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The triangle A340264 without the main diagonal provides a refinement of this sequence.
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KEYWORD
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nonn,nice,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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