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A116508 a(n) = C( C(n,2), n) for n >= 1. 2
0, 0, 1, 15, 252, 5005, 116280, 3108105, 94143280, 3190187286, 119653565850, 4922879481520, 220495674290430, 10682005290753420, 556608279578340080, 31044058215401404845, 1845382436487682488000, 116475817125419611477660 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

COMMENTS

a(n) is the number of simple labeled graphs with n nodes and n edges. - Geoffrey Critzer, Nov 02 2014

LINKS

Alois P. Heinz, Table of n, a(n) for n = 1..200

FORMULA

a(n) ~ exp(n - 2) * n^(n - 1/2) / (sqrt(Pi) * 2^(n + 1/2)). - Vaclav Kotesovec, May 19 2020

EXAMPLE

a(5) = C(C(5,2),5) = C(10,5) = 252.

MAPLE

a:= n-> binomial(binomial(n, 2), n):

seq(a(n), n=1..20);

MATHEMATICA

nn = 18; f[x_, y_] :=

Sum[(1 + y)^Binomial[n, 2] x^n/n!, {n, 1, nn}]; Table[

n! Coefficient[Series[f[x, y], {x, 0, nn}], x^n y^n], {n, 1, nn}] (* Geoffrey Critzer, Nov 02 2014 *)

PROG

(Sage) [(binomial(binomial(n+2, n), n+2)) for n in range(-1, 17)] # Zerinvary Lajos, Nov 30 2009

(MAGMA) [0] cat [(Binomial(Binomial(n+2, n), n+2)): n in [0..20]]; // Vincenzo Librandi, Nov 03 2014

CROSSREFS

Cf. A084546.

Sequence in context: A093147 A218192 A066410 * A055659 A218368 A123816

Adjacent sequences:  A116505 A116506 A116507 * A116509 A116510 A116511

KEYWORD

easy,nonn

AUTHOR

Christopher Hanusa (chanusa(AT)math.binghamton.edu), Mar 21 2006

STATUS

approved

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Last modified May 11 16:48 EDT 2021. Contains 343805 sequences. (Running on oeis4.)