This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A309851 #20 Sep 08 2022 08:46:22
%S A309851 11,23,47,59,1019,1223,1427,1733,2039,2141,2243,2447,2549,2753,2957,
%T A309851 3467,3671,4079,4283,4691,4793,5099,52103,55109,61121,65129,70139,
%U A309851 75149,77153,82163,86171,95189,102203,104207,112223,119237,124247,130259,132263,137273,145289,146291,147293,149297,150299,160319
%N A309851 Primes formed by concatenating n and 2n-1.
%H A309851 Chai Wah Wu, <a href="/A309851/b309851.txt">Table of n, a(n) for n = 1..10000</a>
%t A309851 f[n_] := FromDigits @ (Join @@ IntegerDigits[{n, 2n-1}]); Select[f /@ Range[160], PrimeQ] (* _Amiram Eldar_, Sep 24 2019 *)
%o A309851 (Magma) [a:m in [1..170]|IsPrime(a) where a is 10^(#Intseq(2*m-1))*m+2*m-1]; // _Marius A. Burtea_, Oct 09 2019
%o A309851 (Python)
%o A309851 from sympy import isprime
%o A309851 A309851_list = [m for m in (int(str(n)+str(2*n-1)) for n in range(1,10**5)) if isprime(m)] # _Chai Wah Wu_, Oct 23 2019
%Y A309851 Cf. A000040, A052089 (n and n-1), A030458 (n and n+1), A309808 (n and 2n+1).
%K A309851 nonn,base
%O A309851 1,1
%A A309851 _A.H.M. Smeets_, Aug 20 2019