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

Terms

a(0) =2a(1) =5a(2) =20a(3) =44a(4) =77a(5) =119a(6) =189a(7) =275a(8) =405a(9) =527a(10) =702a(11) =902a(12) =1127a(13) =1430a(14) =1769a(15) =2144a(16) =2555a(17) =3002a(18) =3485a(19) =4094a(20) =4752a(21) =5459a(22) =6215a(23) =7020a(24) =8000a(25) =8910a(26) =10010a(27) =11174a(28) =12402a(29) =13694