OFFSET
1,1
COMMENTS
Partial sums of series of three doubled triangular numbers of the same value in order: 2,2,2,6,6,6,12,12,12,20,20,20... (cf. A002378).
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..1000
Index entries for linear recurrences with constant coefficients, signature (2,-1,2,-4,2,-1,2,-1).
FORMULA
From Colin Barker, Oct 01 2016: (Start)
a(n) = 2*a(n-1)-a(n-2)+2*a(n-3)-4*a(n-4)+2*a(n-5)-a(n-6)+2*a(n-7)-a(n-8) for n>8.
G.f.: 2*x / ((1-x)^4*(1+x+x^2)^2).
(End)
MATHEMATICA
Accumulate[Flatten[{#, #, #}&/@Accumulate[2*Range[30]]]] (* Harvey P. Dale, May 01 2014 *)
PROG
(PARI) Vec(2*x/((1-x)^4*(1+x+x^2)^2) + O(x^60)) \\ Colin Barker, Oct 01 2016
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Jess Tauber, May 14 2013
EXTENSIONS
More terms from N. J. A. Sloane, May 17 2013
STATUS
approved