OFFSET
1,2
COMMENTS
If n == -1 (mod 10^k) then 10^(n*k) divides a(n), so 10^9 divides a(9), 10^19 divides a(19),...,10^198 divides a(99), etc. - Farideh Firoozbakht, Nov 27 2005
LINKS
James East The Work Performed by a Transformation Semigroup, preprint 2005.
FORMULA
(n+1)^n*(n^2-n)/3
EXAMPLE
When n=2 there are 9 partial maps {1,2}->{1,2}: these are (1 1), (2 2), (1 2), (2 1), (1 -), (2 -), (- 1), (- 2) (- -). Adding up the work performed by these maps (from left to right as arranged above) gives a(2)=1+1+0+2+0+1+1+0+0=6.
MATHEMATICA
Table[(n + 1)^n*(n^2 - n)/3, {n, 17}] (* Robert G. Wilson v *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
James East, Nov 23 2005
EXTENSIONS
More terms from Farideh Firoozbakht and Robert G. Wilson v, Nov 27 2005
STATUS
approved