OFFSET
1,6
COMMENTS
The first numbers with this property are 21, 42, 69, 81, 84, 87, 93, and 117. - T. D. Noe, Jun 20 2012
LINKS
T. D. Noe, Table of n, a(n) for n = 1..501
Nina Chen, Recurrence relation.
Index entries for linear recurrences with constant coefficients, signature (1,5,-3,-6).
FORMULA
a(n) = (1/6)*(2^n - (-1)^n - 3^((n+1)/2)). G.f.: x^3 / ((1+x)*(1-2*x)*(1-3*x^2)). - Ralf Stephan, Aug 08 2004
a(n) = a(n-1) + 2 * a(n-2) + 3^(n/2) * (1 + (-1)^n) / 18 for all n in Z. - Michael Somos, Jan 23 2014
a(n) = - 6 * a(n-4) - 3 * a(n-3) + 5 * a(n-2) + a(n-1) for n > 4. - Hugo Pfoertner, Jun 13 2017
EXAMPLE
G.f. = x^4 + x^5 + 6*x^6 + 8*x^7 + 29*x^8 + 45*x^9 + 130*x^10 + 220*x^11 + ...
MATHEMATICA
nn = 35; CoefficientList[Series[x^3/((1 + x) (1 - 2 x) (1 - 3 x^2)), {x, 0, nn}], x] (* T. D. Noe, Jun 20 2012 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
STATUS
approved