OFFSET
1,2
LINKS
Index entries for linear recurrences with constant coefficients, signature (4, -6, 4, -1).
FORMULA
a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) - a(n-4).
a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) + 320.
a(n) = (2*n-1)*(4*n-3)*(20*n-17)/3.
G.f.: x*(1 + 111*x + 191*x^2 + 17*x^3)/(1-x)^4.
EXAMPLE
The sequence of heptagonal pyramidal numbers A002413 begins 1, 8, 26, 60, 115, 196, 308, 456, 645, 880, ... . As the third odd term is 645, a(3) = 645.
MATHEMATICA
LinearRecurrence[{4, -6, 4, -1}, {1, 115, 645, 1911}, 34]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Ant King, Oct 26 2012
STATUS
approved