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 A054257 #8 Oct 20 2019 17:26:55
%S A054257 21,117,241,283,389,517,547,627,1219,1223,1247,1285,1287,1323,1467,
%T A054257 1505,1591,1599,1689,1713,1817,1961,2203,2309,2377,2607,2837,2847,
%U A054257 2899,2911,3227,3261,3499,3823,4007,4069,4183,4347,4403,4473,4507,4535,4637
%N A054257 Concatenation of n in base 10 down up to base 2 is prime, all numbers are interpreted as decimals.
%H A054257 Harvey P. Dale, <a href="/A054257/b054257.txt">Table of n, a(n) for n = 0..1000</a>
%e A054257 a(1)=21 -> 21{10}=23{9}=25{8}=30{7}=33{6}=41{5}=111{4}=210{3}=10101{2} -> 21232530334111121010101 is a prime.
%t A054257 Select[Range[5000],PrimeQ[FromDigits[Flatten[Table[IntegerDigits[#,n],{n,10,2,-1}]]]]&] (* _Harvey P. Dale_, May 25 2012 *)
%Y A054257 Cf. A054256, A054258.
%K A054257 nonn,base
%O A054257 0,1
%A A054257 _Patrick De Geest_, Feb 15 2000