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

Terms

a(0) =0a(1) =0a(2) =2a(3) =5a(4) =9a(5) =13a(6) =19a(7) =26a(8) =34a(9) =42a(10) =52a(11) =63a(12) =75a(13) =87a(14) =101a(15) =116a(16) =132a(17) =148a(18) =166a(19) =185a(20) =205a(21) =225a(22) =247a(23) =270a(24) =294a(25) =318a(26) =344a(27) =371a(28) =399a(29) =427