a(n) = floor(n/3)*floor((n+1)/3).

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =1a(5) =2a(6) =4a(7) =4a(8) =6a(9) =9a(10) =9a(11) =12a(12) =16a(13) =16a(14) =20a(15) =25a(16) =25a(17) =30a(18) =36a(19) =36a(20) =42a(21) =49a(22) =49a(23) =56a(24) =64a(25) =64a(26) =72a(27) =81a(28) =81a(29) =90