a(n) = floor(n*(n+1/2)/4).

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =4a(5) =6a(6) =9a(7) =13a(8) =17a(9) =21a(10) =26a(11) =31a(12) =37a(13) =43a(14) =50a(15) =58a(16) =66a(17) =74a(18) =83a(19) =92a(20) =102a(21) =112a(22) =123a(23) =135a(24) =147a(25) =159a(26) =172a(27) =185a(28) =199a(29) =213