A126950 - OEIS

%I #2 Mar 31 2012 14:40:14

%S 1,15,39,89,161,271,415,609,849,1151,1511,1945,2449,3039,3711,4481,

%T 5345,6319,7399,8601,9921,11375,12959,14689,16561,18591,20775,23129,

%U 25649,28351,31231,34305,37569,41039,44711,48601,52705,57039,61599,66401

%N a(1) = 1; for n>1, a(n) = the smallest number p > a(n-1) such that (a(n-1)+p)/2 is a cube.

%F a(n) = ((2*n +1)*(2*n^2 + 2*n -1)+ 5*(-1)^n)/4; a(n) = a(n-1)+2n^3; G.f. = (1 - 2*x + 14*x^2 - 2*x^3 + x^4)/((1 + x)(1 - x)^4).

%t Table[((2*n +1)*(2*n^2 + 2*n -1)+ 5*(-1)^n)/4,{n,83}]

%Y Cf. A081352, A126938.

%K nonn

%O 1,2

%A _Zak Seidov_, Mar 18 2007