a(n) = n*(n+3)/2.

Terms

a(0) =0a(1) =2a(2) =5a(3) =9a(4) =14a(5) =20a(6) =27a(7) =35a(8) =44a(9) =54a(10) =65a(11) =77a(12) =90a(13) =104a(14) =119a(15) =135a(16) =152a(17) =170a(18) =189a(19) =209a(20) =230a(21) =252a(22) =275a(23) =299a(24) =324a(25) =350a(26) =377a(27) =405a(28) =434a(29) =464