A026052 (d(n)-r(n))/2, where d = A008778 and r is the periodic sequence with fundamental period (1,1,0,1).

2, 6, 13, 22, 35, 52, 74, 100, 132, 170, 215, 266, 325, 392, 468, 552, 646, 750, 865, 990, 1127, 1276, 1438, 1612, 1800, 2002, 2219, 2450, 2697, 2960, 3240, 3536, 3850, 4182, 4533, 4902, 5291, 5700, 6130, 6580, 7052, 7546, 8063, 8602, 9165, 9752, 10364, 11000, 11662, 12350, 13065, 13806, 14575

Offset: 2

Author

Clark Kimberling

Keywords

• nonn

Formula

a(n)=(n + 2)*(n^2 + 10*n + 15)/12 - 0.375 + 0.125*( - 1)^n - 0.25*cos(n*Pi/2) [From Richard Choulet, Dec 13 2008]