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A290011 Number of ways to connect n nodes with n+1 edges to form a 2-edge-connected graph. 1
6, 85, 900, 9450, 104160, 1224720, 15422400, 207900000, 2993760000, 45924278400, 748280332800, 12913284384000, 235381386240000, 4520194398720000, 91233825306624000, 1931115968990208000, 42778526977105920000, 989887004576870400000, 23885015465274163200000 (list; graph; refs; listen; history; text; internal format)
OFFSET

4,1

LINKS

Robert Israel, Table of n, a(n) for n = 4..448

FORMULA

a(n) = (n - 4)*(n!/8) + (n*(n - 1)/2 - 3)*(n!/12) = (n^2 + 2 n - 18)*(n!/24).

E.g.f.: x^4*(3*x^2+x-6)/(24*(x-1)^3). - Robert Israel, Jul 19 2017

MAPLE

seq((n^2 + 2 *n - 18)* n!/24, n=6..30); # Robert Israel, Jul 19 2017

MATHEMATICA

Table[(n - 4) (n!/8) + (n (n - 1)/2 - 3) (n!/12), {n, 4, 22}] (* Michael De Vlieger, Jul 18 2017 *)

PROG

(PARI) a(n) = (n - 4)*(n!/8) + (n*(n - 1)/2 - 3)*(n!/12); \\ Michel Marcus, Jul 18 2017

CROSSREFS

Cf. A001072, A052447, A054596, A095983

Sequence in context: A245232 A284522 A167252 * A164266 A136597 A064329

Adjacent sequences:  A290008 A290009 A290010 * A290012 A290013 A290014

KEYWORD

easy,nonn

AUTHOR

Eugene Y. Q. Shen, Jul 17 2017

STATUS

approved

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Last modified November 18 01:09 EST 2018. Contains 317279 sequences. (Running on oeis4.)