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 A056721 #35 Jun 18 2025 11:24:04
%S A056721 0,1,4,5,8,10,25,49,76,128,175,238,550,796,1219,2012,2846,11336,21296,
%T A056721 49808,74318,236891,608989,1652593,1715905,1990324,2045966,2084563,
%U A056721 5112847
%N A056721 Numbers k such that 8*10^k-1 is prime.
%C A056721 Also numbers n such that 7*10^n + 9*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%H A056721 Makoto Kamada, <a href="https://stdkmd.net/nrr/7/79999.htm#prime">Prime numbers of the form 799...99</a>.
%H A056721 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t A056721 Do[ If[ PrimeQ[ 7*10^n + (10^n-1)], Print[n]], {n, 0, 3000}]
%o A056721 (PARI) is(n)=ispseudoprime(8*10^n-1) \\ _Charles R Greathouse IV_, Feb 20 2017
%K A056721 nonn,hard,more
%O A056721 1,3
%A A056721 _Robert G. Wilson v_, Aug 11 2000
%E A056721 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E A056721 a(22)-a(23) from Kamada data by _Robert Price_, Oct 19 2014
%E A056721 a(24)-a(25) from Kamada data by _Tyler Busby_, Apr 15 2024
%E A056721 Missing terms from _Ryan Propper_, Jun 02 2025