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A247659 Number of down-arrow-indecomposable topologies on n labeled points. 1
1, 3, 22, 292, 6120, 193594, 9070536, 622336756, 61915861962, 8846814822932, 1798543906246948, 515674104905890202, 206833212761446463192, 115198617558900993580396, 88503974769306037986089170, 93233054587165663487254293572 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..16.

L. Foissy and C. Malvenuto, The Hopf algebra of finite topologies and T-partitions, arXiv preprint arXiv:1407.0476, 2014

FORMULA

G.f.: (A000798(x)-1)/A000798(x), where A000798(x) is the g.f. of A000798.

MATHEMATICA

A000798 = {1, 1, 4, 29, 355, 6942, 209527, 9535241, 642779354, 63260289423, 8977053873043, 1816846038736192, 519355571065774021, 207881393656668953041, 115617051977054267807460, 88736269118586244492485121, 93411113411710039565210494095, 134137950093337880672321868725846, 261492535743634374805066126901117203};

f[x_] = Sum[A000798[[n+1]] x^n, {n, 0, nmax = Length[A000798]-1}];

CoefficientList[(f[x]-1)/f[x] + O[x]^nmax, x][[2 ;; -2]] (* Jean-François Alcover, Oct 10 2018 *)

CROSSREFS

Cf. A000598, A247660.

Sequence in context: A135862 A122778 A108991 * A244468 A325295 A298693

Adjacent sequences:  A247656 A247657 A247658 * A247660 A247661 A247662

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Oct 05 2014

STATUS

approved

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Last modified December 6 15:23 EST 2021. Contains 349563 sequences. (Running on oeis4.)