OFFSET
1,1
COMMENTS
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.
LINKS
Samuel Wagstaff, The Cunningham Project
EXAMPLE
a(1) = 7 so 1111111 = 46E * 2X3E (written in base 12).
MATHEMATICA
Select[Range[120], PrimeOmega[(12^# - 1)/11] == 2 &] (* Alonso del Arte, Dec 18 2014 *)
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Eric Chen, Dec 18 2014
STATUS
approved