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 A127996 #22 Aug 20 2021 10:03:04 %S A127996 3,11,17,43,271,156217,328129 %N A127996 Numbers n such that (21^n - 1)/20 is prime. %H A127996 Paul Bourdelais, <a href="https://listserv.nodak.edu/cgi-bin/wa.exe?A2=NMBRTHRY;417ab0d6.0906">A Generalized Repunit Conjecture</a> %H A127996 H. Dubner, <a href="http://dx.doi.org/10.1090/S0025-5718-1993-1185243-9">Generalized repunit primes</a>, Math. Comp., 61 (1993), 927-930. %H A127996 H. Lifchitz, <a href="http://www.primenumbers.net/Henri/us/MersFermus.htm">Mersenne and Fermat primes field</a> %H A127996 <a href="/index/Pri#primepop">Index to primes in various ranges</a>, form ((k+1)^n-1)/k %t A127996 Select[Prime[Range[100]],PrimeQ[(21^#-1)/20]&] %o A127996 (Prime95) PRP=1,21,328129,-1,0,0,"20" %o A127996 (PARI) is(n)=isprime((21^n-1)/20) \\ _Charles R Greathouse IV_, Feb 17 2017 %K A127996 hard,more,nonn %O A127996 1,1 %A A127996 _Alexander Adamchuk_, Feb 11 2007 %E A127996 a(6)=156217 is a probable prime discovered by _Paul Bourdelais_, Apr 26 2010 %E A127996 a(7)=328129 is a probable prime discovered by _Paul Bourdelais_, Jun 02 2014