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 A165243 #11 Apr 01 2019 02:04:42
%S A165243 23,211,223,229,241,271,283,2113,2131,2137,2179,2239,2251,2269,2281,
%T A165243 2293,2311,2347,2383,2389,2467,2503,2521,2557,2593,2617,2647,2659,
%U A165243 2677,2683,2719,2797,2857,2887,2953,2971,21013,21019,21031,21061,21163,21187
%N A165243 Primes obtained from other primes by pre-concatenating with 2.
%e A165243 The prime 2971 is from the prime 971 by pre-concatenating with 2.
%t A165243 Select[Map[FromDigits,
%t A165243 Map[Join[{2}, #] &, IntegerDigits[Prime[Range[200]]]]], PrimeQ[#] &] (* _Robert Price_, Mar 31 2019 *)
%o A165243 (PARI) terms(n) = my(i=0, cc=[]); forprime(p=1, , cc=eval(concat("2", Str(p))); if(ispseudoprime(cc), print1(cc, ", "); i++); if(i==n, break))
%o A165243 /* Print initial 40 terms as follows: */
%o A165243 terms(40) \\ _Felix Fröhlich_, Mar 31 2019
%Y A165243 Cf. A000040
%K A165243 base,nonn
%O A165243 1,1
%A A165243 _Parthasarathy Nambi_, Sep 09 2009
%E A165243 2269 inserted by _R. J. Mathar_, Sep 21 2009