A026038 a(n) = (d(n)-r(n))/2, where d = A026037 and r is the periodic sequence with fundamental period (1,0,0,1).

5, 11, 20, 33, 52, 78, 111, 152, 203, 265, 338, 423, 522, 636, 765, 910, 1073, 1255, 1456, 1677, 1920, 2186, 2475, 2788, 3127, 3493, 3886, 4307, 4758, 5240, 5753, 6298, 6877, 7491, 8140, 8825, 9548, 10310, 11111, 11952, 12835, 13761, 14730, 15743, 16802, 17908, 19061, 20262, 21513, 22815, 24168