OFFSET
0,4
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..1000
FORMULA
G.f.: 2*x^3*(15*x^6-167*x^5+320*x^4+686*x^3-2789*x^2-1815*x-90)/(x-1)^7.
a(n) = (2574-2328*n-144*n^5-96*n^4+1188*n^3-854*n^2+32*n^6)/3 for n>=3, a(n) = 0 for n<3.
EXAMPLE
a(3) = 180:
+-----+ +-----+ +-----+ +-----+
|o-o | | o | | o | | o |
| | | \ | | / | | / |
|o-o | |o o o| |o o | |o o o|
| | || | | | / | | X |
| o-o| |o o | |o o-o| | o o|
+-----+ +-----+ +-----+ +-----+ ... .
MAPLE
a:= n-> `if`(n<3, 0, ((((((32*n-144)*n-96)*n+1188)*n-854)
*n-2328)*n+2574)/3):
seq(a(n), n=0..50);
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Alois P. Heinz, Jun 05 2014
STATUS
approved