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 A347343 #9 Aug 29 2021 02:02:32
%S A347343 1,19,21,23,27,31,43,49,57,59,67,73,81,87,91,97,103,127,139,143,149,
%T A347343 151,169,173,177,181,187,193,199,201,209,211,231,233,237,239,241,247,
%U A347343 263,267,269,271,277,283,299,301,329,343,349,351,353,367,373,383,387
%N A347343 Positive integers k such that k with the last digit repeated is prime.
%H A347343 Michael S. Branicky, <a href="/A347343/b347343.txt">Table of n, a(n) for n = 1..10000</a>
%F A347343 a(n) ~ n log n by the prime number theorem in arithmetic progressions. (These numbers are the primes mod 11, 33, 77, or 99 mod 100 with their last digit removed.) - _Charles R Greathouse IV_, Aug 27 2021
%e A347343 21 is a term because 211 is prime.
%t A347343 upto=500;Select[Range[1,upto,2],PrimeQ[FromDigits[Join[d=IntegerDigits[#],{Last[d]}]]]&]
%o A347343 (PARI) forprime(p=9,1e4, if(p%100%11==0, print1(p\10", "))) \\ _Charles R Greathouse IV_, Aug 27 2021
%o A347343 (Python)
%o A347343 from sympy import isprime
%o A347343 def ok(n): return isprime(10*n + n%10)
%o A347343 print(list(filter(ok, range(388)))) # _Michael S. Branicky_, Aug 28 2021
%Y A347343 Cf. A000040, A226099.
%K A347343 nonn,base,easy
%O A347343 1,2
%A A347343 _Paolo Xausa_, Aug 27 2021