OFFSET
2,2
COMMENTS
Wright also gives the number of strongly connected digraphs with n vertices and n+2 edges, 0, 6, 316, 6440, 107850, 1719060, 27476400, ... (offset 2) in terms of a polynomial of order 5 multiplied by n!. - R. J. Mathar, May 12 2016
LINKS
Andrew Howroyd, Table of n, a(n) for n = 2..200
E. M. Wright, Formulae for the number of sparsely-edged strong labelled digraphs, Quart. J. Math. 28 (3) (1977) 363-367, Section 3.
FORMULA
a(n) = (n-2)*(n+3)*n!/4.
E.g.f.: x^3*(3 - 2*x)/(2*(1 - x)^3). - Ilya Gutkovskiy, May 10 2016
D-finite with recurrence -(n+1)*(n-4)*a(n) +(n-1)*(n-3)*(n+2)*a(n-1)=0. - R. J. Mathar, Mar 11 2021
MATHEMATICA
Table[(n-2)(n+3)n!/4, {n, 2, 30}] (* Harvey P. Dale, May 23 2017 *)
PROG
(Python)
from __future__ import print_function
from sympy import factorial
for n in range(2, 500):
print((int)((n-2)*(n+3)*factorial(n)/4), end=", ")
# Soumil Mandal, May 12 2016
(PARI) a(n) = (n-2)*(n+3)*n!/4 \\ Andrew Howroyd, Jan 15 2022
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
R. J. Mathar, May 10 2016
STATUS
approved