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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). 6
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

Table of n, a(n) for n=1..34.

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: A118408 A079268 A102316 * A173651 A124040 A160332

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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Last modified April 25 00:50 EDT 2014. Contains 240991 sequences.