|
|
A107660
|
|
Sum 3^max(k,n-k),k=0..n.
|
|
2
|
|
|
1, 6, 21, 72, 225, 702, 2133, 6480, 19521, 58806, 176661, 530712, 1592865, 4780782, 14344533, 43040160, 129127041, 387400806, 1162222101, 3486725352, 10460235105, 31380882462, 94142824533, 282429005040, 847287546561
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
0,2
|
|
COMMENTS
|
|
|
LINKS
|
|
|
FORMULA
|
G.f.: (1+3*x)/((1-3*x)*(1-3*x^2)).
a(2n) = 3^(2n+1) - 2*3^n; a(2n+1) = 9^(n+1) - 3^(n+1).
|
|
MATHEMATICA
|
CoefficientList[Series[(1 + 3 x) / ((1 - 3 x) (1 - 3 x^2)), {x, 0, 40}], x] (* Vincenzo Librandi, Aug 17 2013 *)
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn,easy
|
|
AUTHOR
|
|
|
STATUS
|
approved
|
|
|
|