First unknown term is 311.
If (12^n - 1)/11 is a semiprime, n must be prime or the square of a prime (
A001248), but no n = prime squared is known which yields a semiprime value of (12^n - 1)/11. (Specifically, n must be the square of a prime in
A004064, and must be at least 491401 = 701^2.)
No other known terms below 1000; the only other possible terms below 1000 are 449, 521, 571, 577, 613, 709, 751, 757, 769, 787, 853, 859, 887, 929, and 991.
