a(n) is the number of solutions to x+y+z = 0 mod 3, where 1 <= x < y < z <= n.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =2a(5) =4a(6) =8a(7) =13a(8) =20a(9) =30a(10) =42a(11) =57a(12) =76a(13) =98a(14) =124a(15) =155a(16) =190a(17) =230a(18) =276a(19) =327a(20) =384a(21) =448a(22) =518a(23) =595a(24) =680a(25) =772a(26) =872a(27) =981a(28) =1098a(29) =1224