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 A103001 #27 Jul 24 2025 14:10:11
%S A103001 1,2,4,7,24,172,173,218,439,2563,2782,2880,3715,4392,10820,14592,
%T A103001 25054,27769,40168,180992,193907
%N A103001 Numbers k such that 5*10^k - 9 is prime.
%C A103001 Also numbers k such that 4*10^k + 9*R_k - 8 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A103001 a(22) > 2*10^5. - _Robert Price_, May 30 2015
%H A103001 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/49991.htm#prime">Prime numbers of the form 499...991</a>.
%H A103001 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A103001 a(n) = A101738(n) + 1.
%t A103001 Do[ If[ PrimeQ[5*10^n - 9], Print[n]], {n, 0, 10000}]
%o A103001 (PARI) is(n)=ispseudoprime(5*10^n-9) \\ _Charles R Greathouse IV_, Jun 06 2017
%Y A103001 Cf. A002275, A101738.
%K A103001 more,nonn
%O A103001 1,2
%A A103001 _Robert G. Wilson v_, Jan 17 2005
%E A103001 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E A103001 a(17)-a(19) from Kamada data added by _Robert Price_, Dec 10 2010
%E A103001 a(20)-a(21) from _Robert Price_, May 30 2015