OFFSET
0,2
LINKS
OEIS Wiki, Figurate numbers
Eric Weisstein's World of Mathematics, Octagonal Number
Index entries for linear recurrences with constant coefficients, signature (6,-15,20,-15,6,-1)
FORMULA
O.g.f.: (1 + 5*x)^2/(1 - x)^6.
E.g.f.: (30 + 450*x + 1125*x^2 + 725*x^3 + 150*x^4 + 9*x^5)*exp(x)/30.
a(n) = 6*a(n-1) - 15*a(n-2) + 20*a(n-3) - 15*a(n-4) + 6*a(n-5) - a(n-6).
a(n) = (n + 1)*(n + 2)*(n + 3)*(9*n^2 + 6*n + 5)/30.
MATHEMATICA
LinearRecurrence[{6, -15, 20, -15, 6, -1}, {1, 16, 106, 416, 1211, 2912}, 36]
Table[(n + 1) (n + 2) (n + 3) ((9 n^2 + 6 n + 5)/30), {n, 0, 35}]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Ilya Gutkovskiy, Sep 05 2016
STATUS
approved