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 A034816 #5 Oct 06 2019 18:19:02 %S A034816 9,51,63,117,123,157,183,213,217,219,257,263,321,327,333,423,441,473, %T A034816 541,597,621,687,693,723,777,879,909,987,1101,1143,1187,1387,1459, %U A034816 1551,1887,2127,2179,2193,2817,3197,3339,3513,3633,4029,4209,4429,4529,4557 %N A034816 Concatenations C1 and C2 are both prime (see the comment lines). %C A034816 C1 = 'prevprime(n) followed by n' %C A034816 C2 = 'n followed by nextprime(n)' %e A034816 n=1551 -> previous prime is 1549, next prime is 1553, thus '15491551' and '15511553' are both primes. %Y A034816 Cf. A034808-A034821. %K A034816 nonn,base %O A034816 0,1 %A A034816 _Patrick De Geest_, Oct 15 1998