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Numbers that are the sum of two centered triangular numbers (A005448).
2

%I #9 Jul 26 2017 11:50:41

%S 2,5,8,11,14,20,23,29,32,35,38,41,47,50,56,62,65,68,74,77,83,86,89,92,

%T 95,104,110,113,116,119,128,131,137,140,146,149,155,167,170,173,176,

%U 182,185,194,197,200,203,209,212,218,221,230,236,239,245,251,254,263,266,272,275,278,281,284,293,299

%N Numbers that are the sum of two centered triangular numbers (A005448).

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/CenteredTriangularNumber.html">Centered Triangular Number</a>

%H <a href="/index/Ce#CENTRALCUBE">Index entries for sequences related to centered polygonal numbers</a>

%F 8*a(n) = 10+3*A097269(n). - _R. J. Mathar_, Jul 26 2017

%t nmax = 300; f[x_] := Sum[x^(3 k (k - 1)/2 + 1), {k, 1, 20}]^2; Exponent[#, x] & /@ List @@ Normal[Series[f[x], {x, 0, nmax}]]

%Y Cf. A005448, A020756, A051533, A102795, A286636.

%K nonn

%O 1,1

%A _Ilya Gutkovskiy_, Jun 03 2017