OFFSET
0,2
COMMENTS
Sum of n-th 9-gonal (nonagonal) number and n-th 10-gonal (decagonal) number.
Sum of reciprocals of a(n), for n > 0: 0.614629940137818703272919217222307...
LINKS
Bruno Berselli, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (3,-3,1).
FORMULA
G.f.: x*(2+13*x)/(1-x)^3.
a(n) + a(-n) = A064761(n).
From Elmo R. Oliveira, Jan 12 2025: (Start)
E.g.f.: exp(x)*x*(4 + 15*x)/2.
a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) for n > 2. (End)
MATHEMATICA
Table[n (15 n - 11)/2, {n, 0, 50}]
CoefficientList[Series[x (2 + 13 x) / (1 - x)^3, {x, 0, 45}], x] (* Vincenzo Librandi, Aug 18 2013 *)
PROG
(Magma) [n*(15*n-11)/2: n in [0..50]];
(Magma) I:=[0, 2, 19]; [n le 3 select I[n] else 3*Self(n-1)-3*Self(n-2)+Self(n-3): n in [1..45]]; // Vincenzo Librandi, Aug 18 2013
(PARI) a(n)=n*(15*n-11)/2 \\ Charles R Greathouse IV, Oct 07 2015
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Bruno Berselli, Jun 09 2013
STATUS
approved