A290011 Number of ways to connect n nodes with n+1 edges to form a 2-edge-connected graph.
6, 85, 900, 9450, 104160, 1224720, 15422400, 207900000, 2993760000, 45924278400, 748280332800, 12913284384000, 235381386240000, 4520194398720000, 91233825306624000, 1931115968990208000, 42778526977105920000, 989887004576870400000, 23885015465274163200000
Offset: 4
Links
- Robert Israel, Table of n, a(n) for n = 4..448
Programs
-
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 *) -
PARI
a(n) = (n - 4)*(n!/8) + (n*(n - 1)/2 - 3)*(n!/12); \\ Michel Marcus, Jul 18 2017
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