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 A133709 Triangle read by rows: T(m,l) = number of labeled covers of size l of a finite set of m unlabeled elements (m >= 1, 1 <= l <= 2^m - 1). 7
 1, 1, 3, 3, 1, 7, 35, 140, 420, 840, 840, 1, 12, 131, 1435, 15225, 150570, 1351770, 10810800, 75675600, 454053600, 2270268000, 9081072000, 27243216000, 54486432000, 54486432000, 1, 18, 347, 7693, 185031, 4568046, 111793710, 2661422400 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 LINKS A. P. Burger and J. H. van Vuuren, Balanced minimal covers of a finite set, Discr. Math. 307 (2007), 2853-2860. FORMULA Burger and van Vuuren give an explicit formula. EXAMPLE Triangle begins: 1 1 3 3 1 7 35 140 420 840 840 1 12 131 1435 15225 150570 1351770 MAPLE A133709 := proc(m, l)         option remember;         if l = 1 then                 1;         else                 add((-1)^i*binomial(l, i)*binomial(2^(l-i)+m-2, m), i=0..l-1)                 - add(combinat[stirling2](l, i)*procname(m, i), i=1..l-1) ;         end if; end proc: seq(seq(A133709(m, l), l=1..2^m-1), m=1..5) ; # R. J. Mathar, Nov 23 2011 CROSSREFS Columns are given by A055998, A133710-A133712. Sequence in context: A079268 A102316 A261767 * A173651 A124040 A248976 Adjacent sequences:  A133706 A133707 A133708 * A133710 A133711 A133712 KEYWORD nonn,tabf AUTHOR N. J. A. Sloane, Dec 30 2007 STATUS approved

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