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 A058681 Number of matroids of rank 2 on n labeled points. 28
 0, 0, 1, 7, 36, 171, 813, 4012, 20891, 115463, 677546, 4211549, 27640341, 190891130, 1382942161, 10480109379, 82864804268, 682076675087, 5832741942913, 51724157711084, 474869815108175, 4506715736350171, 44152005850890042 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS 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 Equals row sums of triangle A180338. - Gary W. Adamson, Aug 28 2010 LINKS T. D. Noe, Table of n, a(n) for n = 0..100 W. M. B. Dukes, Tables of matroids. W. M. B. Dukes, Counting and Probability in Matroid Theory, Ph.D. Thesis, Trinity College, Dublin, 2000. W. M. B. Dukes, On the number of matroids on a finite set, arXiv:math/0411557 [math.CO], 2004. A. O. Munagi, k-Complementing Subsets of Nonnegative Integers, International Journal of Mathematics and Mathematical Sciences, 2005:2 (2005), 215-224. FORMULA 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 a(n) = Sum_{i=2..n} binomial(n,i)*(B(i)-1), B=Bell numbers A000110. - Geoffrey Critzer, Oct 10 2009 E.g.f.: exp(x + exp(x) - 1) - exp(2*x). - Peter Luschny, Jan 08 2021 EXAMPLE 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 MAPLE 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 MATHEMATICA f[n_] := Sum[ StirlingS2[n + 1, k+2], {k, 1, n}]; Table[ f[n], {n, 0, 23}] - Zerinvary Lajos, Mar 21 2007 Table[BellB[n+1]-2^n, {n, 0, 30}] (* Harvey P. Dale, Oct 13 2011 *) CROSSREFS Column k = 2 of A058669. The triangle A340264 without the main diagonal provides a refinement of this sequence. Cf. A180338, A005465. Sequence in context: A038748 A099455 A102053 * A246417 A110310 A054493 Adjacent sequences:  A058678 A058679 A058680 * A058682 A058683 A058684 KEYWORD nonn,nice,easy AUTHOR N. J. A. Sloane, Dec 30 2000 EXTENSIONS More terms from James A. Sellers, Jan 03 2001 a(0) = a(1) = 0 prepended by Peter Luschny, Jan 08 2021 STATUS approved

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Last modified May 9 05:09 EDT 2021. Contains 343688 sequences. (Running on oeis4.)