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 A104873 #10 Jan 18 2021 16:04:52 %S A104873 611000000000061,1631000000000163,1931000000000193,2111000000000211, %T A104873 2711000000000271,3311000000000331,5471000000000547,6611000000000661, %U A104873 7511000000000751,7871000000000787,9971000000000997,10511000000001051 %N A104873 Concatenations of pairs of primes that differ by 10^12. %C A104873 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(177) concatenated with Prime(37607912056) = 10511000000001051 = 3 * 3503666666667017. %H A104873 Harvey P. Dale, <a href="/A104873/b104873.txt">Table of n, a(n) for n = 1..1000</a> %F A104873 a(n) = Concatenate(P, P+10^12) iff P prime and P+10^12 prime. %e A104873 61 is prime, specifically prime(18) and 61 + 10^12 is prime, specifically prime(7607912020), so their concatenation is in this sequence: 611000000000061. The concatenation is not itself prime, as it equals 3 * 7 * 23 * 1265010351967. %t A104873 #*10^13+10^12+#&/@Select[Prime[Range[200]],PrimeQ[#+10^12]&] (* _Harvey P. Dale_, Jan 18 2021 *) %Y A104873 Cf. A000040, A001358, A023201, A100750, A103195, A103206, A104718, A104719, A103523, A103534, A103576, A103617. %K A104873 base,easy,nonn %O A104873 1,1 %A A104873 _Jonathan Vos Post_, Mar 29 2005