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%I #10 Jan 18 2021 16:04:52
%S 611000000000061,1631000000000163,1931000000000193,2111000000000211,
%T 2711000000000271,3311000000000331,5471000000000547,6611000000000661,
%U 7511000000000751,7871000000000787,9971000000000997,10511000000001051
%N Concatenations of pairs of primes that differ by 10^12.
%C 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 Harvey P. Dale, <a href="/A104873/b104873.txt">Table of n, a(n) for n = 1..1000</a>
%F a(n) = Concatenate(P, P+10^12) iff P prime and P+10^12 prime.
%e 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 #*10^13+10^12+#&/@Select[Prime[Range[200]],PrimeQ[#+10^12]&] (* _Harvey P. Dale_, Jan 18 2021 *)
%Y Cf. A000040, A001358, A023201, A100750, A103195, A103206, A104718, A104719, A103523, A103534, A103576, A103617.
%K base,easy,nonn
%O 1,1
%A _Jonathan Vos Post_, Mar 29 2005