OFFSET
0,2
LINKS
Index entries for linear recurrences with constant coefficients, signature (1,3,2,-2)
FORMULA
G.f.: (1 + x - x^2)/(1 - x -3*x^2 - 2*x^3 + 2*x^4).
a(n) = a(n-1) +3*a(n-2) +2*a(n-3) -2*a(n-4). - Fung Lam, May 18 2014
EXAMPLE
a(3)=12 because we have: 111, 122, 131, 144, 221, 223, 313, 322, 333, 344, 441, 443.
MATHEMATICA
n=4; nn=30; CoefficientList[Series[1/(1-Sum[v[i]/(1+v[i]), {i, 1, n}])/.Join[Table[v[i]->z/(1-z^2), {i, 1, n, 2}], Table[v[i]->z^2/(1-z^2), {i, 2, n, 2}]], {z, 0, nn}], z]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Geoffrey Critzer, May 17 2014
STATUS
approved