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%I A309808 #18 Sep 08 2022 08:46:22
%S A309808 13,37,613,919,1021,1123,1327,1429,1531,2143,2347,2551,2857,3061,3163,
%T A309808 3469,3571,3673,3877,4591,4999,50101,56113,59119,63127,70141,71143,
%U A309808 74149,78157,79159,81163,88177,91183,93187,95191,101203,105211,106213,108217,110221,113227,114229
%N A309808 Primes formed by concatenating k and 2k+1.
%C A309808 Primes in A309809.
%H A309808 Robert Israel, <a href="/A309808/b309808.txt">Table of n, a(n) for n = 1..10000</a>
%e A309808 a(3)=613 is the concatenation of 6 and 2*6+1=13 and is prime.
%p A309808 select(isprime, [seq(n*10^(1+ilog10(2*n+1))+2*n+1,n=1..200)]);
%o A309808 (Magma) [a:m in [1..130]|IsPrime(a) where a is 10^(#Intseq(2*m+1))*m+2*m+1]; // _Marius A. Burtea_, Aug 18 2019
%Y A309808 Cf. A030458, A309809.
%K A309808 nonn,base
%O A309808 1,1
%A A309808 _J. M. Bergot_ and _Robert Israel_, Aug 17 2019

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