Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #18 Sep 08 2022 08:46:22
%S 13,37,613,919,1021,1123,1327,1429,1531,2143,2347,2551,2857,3061,3163,
%T 3469,3571,3673,3877,4591,4999,50101,56113,59119,63127,70141,71143,
%U 74149,78157,79159,81163,88177,91183,93187,95191,101203,105211,106213,108217,110221,113227,114229
%N Primes formed by concatenating k and 2k+1.
%C Primes in A309809.
%H Robert Israel, <a href="/A309808/b309808.txt">Table of n, a(n) for n = 1..10000</a>
%e a(3)=613 is the concatenation of 6 and 2*6+1=13 and is prime.
%p select(isprime, [seq(n*10^(1+ilog10(2*n+1))+2*n+1,n=1..200)]);
%o (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 Cf. A030458, A309809.
%K nonn,base
%O 1,1
%A _J. M. Bergot_ and _Robert Israel_, Aug 17 2019