Composite k such that sigma(k)-phi(k) is prime.

Terms

a(0) =4a(1) =8a(2) =9a(3) =16a(4) =25a(5) =32a(6) =36a(7) =50a(8) =81a(9) =121a(10) =128a(11) =225a(12) =256a(13) =324a(14) =529a(15) =576a(16) =625a(17) =729a(18) =841a(19) =1058a(20) =1089a(21) =1296a(22) =1681a(23) =1682a(24) =2025a(25) =2312a(26) =2401a(27) =2809a(28) =2916a(29) =3362