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 A034820 #5 Oct 06 2019 18:17:28 %S A034820 16776,41719,164612,188435,188682,312184,317594,392771,397617,450413, %T A034820 476055,486283,492240,497913,539471,584029,620029,640883,648445, %U A034820 656757,903009,992790,993475,995917,1045387,1082078,1194606,1252496,1322841 %N A034820 Concatenations C1 and C2 and C3 and C4 are all prime (see the comment lines). %C A034820 C1 = 'prevprime(n) followed by n followed by prevprime(n)' %C A034820 C2 = 'prevprime(n) followed by n followed by nextprime(n)' %C A034820 C3 = 'nextprime(n) followed by n followed by prevprime(n)' %C A034820 C4 = 'nextprime(n) followed by n followed by nextprime(n)' %e A034820 n=41719 -> next prime is 41729, previous prime is 41687, thus '416874171941687' and '416874171941729' and '417294171941687' and '417294171941729' are all four primes. %Y A034820 Cf. A034808-A034821. %K A034820 nonn,base %O A034820 0,1 %A A034820 _Patrick De Geest_, Oct 15 1998