a(n) = m*(m+1)/2, where m = floor(n^(3/2)).

Terms

a(0) =1a(1) =3a(2) =15a(3) =36a(4) =66a(5) =105a(6) =171a(7) =253a(8) =378a(9) =496a(10) =666a(11) =861a(12) =1081a(13) =1378a(14) =1711a(15) =2080a(16) =2485a(17) =2926a(18) =3403a(19) =4005a(20) =4656a(21) =5356a(22) =6105a(23) =6903a(24) =7875a(25) =8778a(26) =9870a(27) =11026a(28) =12246a(29) =13530