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 A034819 #5 Oct 06 2019 18:17:40 %S A034819 51,63,123,597,687,987,4429,12237,18291,20013,29953,31863,34653,48023, %T A034819 52703,57147,59337,62097,66727,71733,81263,86589,101211,105501,118617, %U A034819 121397,134013,146191,147151,155937,174723,175797,179191,180189,205287 %N A034819 Concatenations C1 and C2 and C3 and C4 are all prime (see the comment lines). %C A034819 C1 = 'nextprime(n) followed by n' %C A034819 C2 = 'n followed by nextprime(n)' %C A034819 C3 = 'prevprime(n) followed by n' %C A034819 C4 = 'n followed by prevprime(n)' %e A034819 n=105501 -> next prime is 105503, previous prime is 105499, thus '105503105501' and '105501105503' and '105501105499' and '105499105501' are all four primes. %Y A034819 Cf. A034808-A034821. %K A034819 nonn,base %O A034819 0,1 %A A034819 _Patrick De Geest_, Oct 15 1998