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A001520
a(n) = (6*n+1)*(6*n+3)*(6*n+5).
1
15, 693, 3315, 9177, 19575, 35805, 59163, 90945, 132447, 184965, 249795, 328233, 421575, 531117, 658155, 803985, 969903, 1157205, 1367187, 1601145, 1860375, 2146173, 2459835, 2802657, 3175935, 3580965, 4019043, 4491465, 4999527, 5544525, 6127755, 6750513
OFFSET
0,1
FORMULA
From Jaume Oliver Lafont, Oct 20 2009: (Start)
G.f.: 3*(1+x)*(5+206*x+5*x^2)/(1-x)^4.
Sum_{k>=0} 1/a(k) = log(3)/16. (End)
a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) - a(n-4) for n >= 4. - Jinyuan Wang, Mar 10 2020
Sum_{n>=0} (-1)^n/a(n) = Pi/48. - Amiram Eldar, Sep 22 2022
MAPLE
A001520:=n->(6*n+1)*(6*n+3)*(6*n+5); seq(A001520(n), n=0..50); # Wesley Ivan Hurt, Mar 10 2014
MATHEMATICA
Table[(6n+1)(6n+3)(6n+5), {n, 0, 50}] (* Wesley Ivan Hurt, Mar 10 2014 *)
PROG
(PARI) a(n)=(6*n+1)*(6*n+3)*(6*n+5) \\ Charles R Greathouse IV, Oct 18 2022
CROSSREFS
Sequence in context: A266519 A177230 A209499 * A062079 A062032 A204251
KEYWORD
nonn,easy
STATUS
approved