Sum of all the middle parts in the partitions of 3n into 3 parts.

Terms

a(0) =1a(1) =5a(2) =18a(3) =40a(4) =80a(5) =135a(6) =217a(7) =320a(8) =459a(9) =625a(10) =836a(11) =1080a(12) =1378a(13) =1715a(14) =2115a(15) =2560a(16) =3077a(17) =3645a(18) =4294a(19) =5000a(20) =5796a(21) =6655a(22) =7613a(23) =8640a(24) =9775a(25) =10985a(26) =12312a(27) =13720a(28) =15254a(29) =16875