OFFSET
2,3
REFERENCES
R. H. Jeurissen and Th. Bezembinder, Enumerating oriented triangle graphs, Ars Comb. 42(1996), 151-160.
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 2..30
FORMULA
a(n) = Sum_{d|n} { d^d * ( (n-1)! )^d / ( n^d * d!) }, valid for n > 4.
EXAMPLE
n=5: 4!/5 + 5^5*(4!)^5 / (5^5*5!) = 66360.
MATHEMATICA
Join[{1, 1, 18},
Table[Plus @@
Map[Function[d, d^d ((n - 1)!)^d /( (n^d) d!)], Divisors[n]], {n, 5, 8}]]
PROG
(PARI) a(n) = sumdiv(n, d, d^d * ( (n-1)! )^d / ( n^d * d!) ); \\ for n>4, Michel Marcus, Mar 29 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Jeurissen (jeuris(AT)sci.kun.nl)
EXTENSIONS
Edited by Olivier Gérard, Mar 29 2014
Corrected and extended by Vincenzo Librandi, Mar 29 2014
STATUS
approved