a(n) = (-1) * round(j((1 + sqrt(-n))/2)), where j is j-invariant.

Terms

a(0) =-1728a(1) =-233a(2) =0a(3) =90a(4) =539a(5) =1539a(6) =3375a(7) =6511a(8) =11663a(9) =19897a(10) =32768a(11) =52512a(12) =82306a(13) =126616a(14) =191658a(15) =286008a(16) =421407a(17) =613808a(18) =884736a(19) =1263051a(20) =1787217a(21) =2508208a(22) =3493226a(23) =4830420a(24) =6634880a(25) =9056199a(26) =12288000a(27) =16579887a(28) =22252407a(29) =29715715