a(n) = smallest k > 1 such that k-1 and k+1 together have n prime divisors.

Terms

a(0) =2a(1) =4a(2) =3a(3) =5a(4) =7a(5) =15a(6) =17a(7) =31a(8) =65a(9) =129a(10) =127a(11) =449a(12) =511a(13) =2561a(14) =1025a(15) =7937a(16) =12799a(17) =20481a(18) =8191a(19) =28673a(20) =65537a(21) =131071a(22) =458751a(23) =360449a(24) =966655a(25) =524287a(26) =4194303a(27) =2097151a(28) =29360129a(29) =34865153