a(1)=4; if n = Product p_i^e_i, n > 1, then a(n) = Product p_{i+1}^{e_i+1}.

Terms

a(0) =4a(1) =9a(2) =25a(3) =27a(4) =49a(5) =225a(6) =121a(7) =81a(8) =125a(9) =441a(10) =169a(11) =675a(12) =289a(13) =1089a(14) =1225a(15) =243a(16) =361a(17) =1125a(18) =529a(19) =1323a(20) =3025a(21) =1521a(22) =841a(23) =2025a(24) =343a(25) =2601a(26) =625a(27) =3267a(28) =961a(29) =11025