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 A120376 #20 Jun 21 2020 01:55:25
%S A120376 1249,31249,305175781249,119209289550781249,1862645149230957031249,
%T A120376 111022302462515654042363166809082031249,
%U A120376 25243548967072377773175314089049159349542605923488736152648925781249
%N A120376 Primes of the form 2*5^k - 1.
%C A120376 See comments for A057472. Examined in base 12, all n must be even and all primes must be 1-primes. For example, 1249 is 881 in base 12.
%C A120376 The next term has 125 digits. - _Harvey P. Dale_, Jan 26 2019
%F A120376 a(n) = 2*5^A120375(n) - 1 = 2*5^(2*A002958(n)) - 1. - _Jianing Song_, Sep 22 2018
%e A120376 a(1) = 4 since 2*5^4 - 1 = 1249 is the first prime.
%p A120376 for w to 1 do for k from 1 to 2000 do n:=2*5^k-1; if isprime(n) then printf("%d, %d",k,n) fi od od;
%t A120376 Select[2*5^Range[100]-1,PrimeQ] (* _Harvey P. Dale_, Jan 26 2019 *)
%o A120376 (PARI) for(k=1, 1e3, if(ispseudoprime(p=2*5^k-1), print1(p, ", "))); \\ _Altug Alkan_, Sep 22 2018
%Y A120376 Integers k such that 2*b^k - 1 is prime: A090748 (b=2), A003307 (b=3), A120375 (b=5), A057472 (b=6), A002959 (b=7), A002957 (b=10), A120378 (b=11).
%Y A120376 Primes of the form 2*b^k - 1: A000668 (b=2), A079363 (b=3), this sequence (b=5), A158795 (b=7), A055558 (b=10), A120377 (b=11).
%Y A120376 Cf. also A000043, A002958.
%K A120376 nonn
%O A120376 1,1
%A A120376 _Walter Kehowski_, Jun 28 2006