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 A120377 #20 Jul 27 2025 08:42:32
%S A120377 241,428717761
%N A120377 Primes of the form 2*11^k-1.
%C A120377 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 A120377 The values of k < 1000 that yield primes are 2, 8, 248. - _T. D. Noe_, Nov 16 2006
%H A120377 Amiram Eldar, <a href="/A120377/b120377.txt">Table of n, a(n) for n = 1..3</a>
%F A120377 a(n) = n-th number such that 2*11^k-1 that is prime for some k.
%F A120377 a(n) = 2*11^A120378(n)-1. - _R. J. Mathar_, Mar 06 2010
%e A120377 a(1) = 241 since 2*11^2-1 = 241 is the first prime.
%p A120377 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 A120377 Select[2*11^Range[1000]-1, PrimeQ] (* _T. D. Noe_, Nov 16 2006 *)
%Y A120377 Cf. A000043, A000668, A002957, A002959, A003307, A055558, A079363, A120378.
%K A120377 nonn,hard,bref
%O A120377 1,1
%A A120377 _Walter Kehowski_, Jun 28 2006
%E A120377 Corrected by _T. D. Noe_, Nov 16 2006