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 A103617 #9 May 14 2022 12:42:59 %S A103617 71000000007,971000000097,1031000000103,1811000000181,2231000000223, %T A103617 2411000000241,2711000000271,3491000000349,4091000000409, %U A103617 4331000000433,4391000000439,6071000000607,6131000000613,7871000000787,8291000000829 %N A103617 Concatenations of pairs of primes that differ by 10^9. %C A103617 Integers in this sequence can never be prime, as they are all multiples of 3. They can be semiprimes, as is the case for Prime(42) concatenated with Prime(50847544) = 1811000000181 = 3 x 603666666727. %H A103617 Harvey P. Dale, <a href="/A103617/b103617.txt">Table of n, a(n) for n = 1..1000</a> %F A103617 a(n) = Concatenate(P, P+1000000000) iff P prime and P+1000000000 prime. %e A103617 181 is prime, 181+10^9 = 1000000181 is prime, so their concatenation is an element of this sequence: 1811000000181. Coincidentally, prime(181)+10^9 = 1000001087 is also prime. %t A103617 FromDigits[Join[IntegerDigits[#],IntegerDigits[#+10^9]]]&/@Select[Prime[ Range[ 200]],PrimeQ[ #+ 10^9]&] (* _Harvey P. Dale_, May 14 2022 *) %Y A103617 Cf. A000040, A001358, A023201, A100750, A103195, A103206, A104718, A104719, A103523, A103534, A103576. %K A103617 base,easy,nonn %O A103617 1,1 %A A103617 _Jonathan Vos Post_, Mar 25 2005