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%I #5 Mar 30 2012 18:40:27
%S 31000003,371000037,1511000151,1931000193,1991000199,2111000211,
%T 3131000313,3671000367,3971000397,4091000409,4571000457,5411000541,
%U 5471000547,5771000577,6191000619,6911000691,8291000829,8591000859
%N Concatenations of pairs of primes that differ by 1000000.
%C 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 a(n) = Concatenate(P, P+1000000) iff P prime and P+1000000 prime.
%e Prime(47) = 211 and 211 + 1000000 = Prime(78515) = 1000211. Concatenating these two primes gives 2111000211 = 3^4 * 17^2 * 31 * 2909.
%Y Cf. A000040, A001358, A023201, A100750, A103195, A103206, A104718, A104719, A103523, A103534.
%K base,easy,nonn
%O 1,1
%A _Jonathan Vos Post_, Mar 23 2005
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