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 A103576 #5 Mar 30 2012 18:40:27 %S A103576 31000003,371000037,1511000151,1931000193,1991000199,2111000211, %T A103576 3131000313,3671000367,3971000397,4091000409,4571000457,5411000541, %U A103576 5471000547,5771000577,6191000619,6911000691,8291000829,8591000859 %N A103576 Concatenations of pairs of primes that differ by 1000000. %C A103576 After the first element, 31000003, which is prime, integers in this sequence can never be prime, as they are all multiples of 3. They can be semiprimes, as is the case for 3671000367 = 3 x 1223666789, 4571000457 = 3 x 1523666819, 5411000541 = 3 x 1803666847, 9071000907 = 3 x 3023666969. %F A103576 a(n) = Concatenate(P, P+1000000) iff P prime and P+1000000 prime. %e A103576 Prime(47) = 211 and 211 + 1000000 = Prime(78515) = 1000211. Concatenating these two primes gives 2111000211 = 3^4 * 17^2 * 31 * 2909. %Y A103576 Cf. A000040, A001358, A023201, A100750, A103195, A103206, A104718, A104719, A103523, A103534. %K A103576 base,easy,nonn %O A103576 1,1 %A A103576 _Jonathan Vos Post_, Mar 23 2005