This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A120378 #12 Nov 07 2015 09:19:06
%S A120378 2,8,248,2474,2900,6600,24746,105704
%N A120378 Integers n such that 2*11^n-1 is prime.
%C A120378 See comments for A057472. Examined in base 12, all n must be even and all primes must be 1-primes. For example, 241 is 181 in base 12.
%C A120378 a(9) > 2*10^5. - _Robert Price_, Nov 06 2015
%F A120378 a(n) = n-th integer k such that 2*11^k-1 is prime.
%e A120378 a(1)=2 since 2*11^2-1=241 is the first prime of this form.
%p A120378 for w to 1 do for k from 1 to 2000 do n:=2*11^k-1; if isprime(n) then printf("%d, %d",k,n) fi od od;
%t A120378 Select[Range[0, 200000], PrimeQ[2*11^# - 1] &] (* _Robert Price_, Nov 06 2015 *)
%Y A120378 Cf. A000043, A000668, A002957, A002959, A003307, A079363, A055558.
%K A120378 more,nonn
%O A120378 1,1
%A A120378 _Walter Kehowski_, Jun 28 2006
%E A120378 More terms from _Ryan Propper_, Jan 14 2008
%E A120378 a(7)-a(8) from _Robert Price_, Nov 06 2015