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%I #9 May 14 2022 12:42:59
%S 71000000007,971000000097,1031000000103,1811000000181,2231000000223,
%T 2411000000241,2711000000271,3491000000349,4091000000409,
%U 4331000000433,4391000000439,6071000000607,6131000000613,7871000000787,8291000000829
%N Concatenations of pairs of primes that differ by 10^9.
%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(42) concatenated with Prime(50847544) = 1811000000181 = 3 x 603666666727.
%H Harvey P. Dale, <a href="/A103617/b103617.txt">Table of n, a(n) for n = 1..1000</a>
%F a(n) = Concatenate(P, P+1000000000) iff P prime and P+1000000000 prime.
%e 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 FromDigits[Join[IntegerDigits[#],IntegerDigits[#+10^9]]]&/@Select[Prime[ Range[ 200]],PrimeQ[ #+ 10^9]&] (* _Harvey P. Dale_, May 14 2022 *)
%Y Cf. A000040, A001358, A023201, A100750, A103195, A103206, A104718, A104719, A103523, A103534, A103576.
%K base,easy,nonn
%O 1,1
%A _Jonathan Vos Post_, Mar 25 2005