a(n) = binomial(n^2,3)/(2*n).

Terms

a(0) =0a(1) =1a(2) =14a(3) =70a(4) =230a(5) =595a(6) =1316a(7) =2604a(8) =4740a(9) =8085a(10) =13090a(11) =20306a(12) =30394a(13) =44135a(14) =62440a(15) =86360a(16) =117096a(17) =156009a(18) =204630a(19) =264670a(20) =338030a(21) =426811a(22) =533324a(23) =660100a(24) =809900a(25) =985725a(26) =1190826a(27) =1428714a(28) =1703170a(29) =2018255