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 A276200 #11 Feb 16 2025 08:33:36
%S A276200 11,113,1231,12343,123449,1234547,12345653,123456761,12345678899,
%T A276200 1234567891003,123456789101099,12345678910111207,1234567891011121309,
%U A276200 123456789101112131383,12345678910111213141337,1234567891011121314151561,123456789101112131415161717,12345678910111213141516171723
%N A276200 Largest prime < the concatenation of the numbers from 1 to n (A007908).
%H A276200 Robert Israel, <a href="/A276200/b276200.txt">Table of n, a(n) for n = 2..368</a>
%H A276200 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PreviousPrime.html">Previous Prime</a>
%F A276200 a(n) = A151799(A007908(n)).
%F A276200 a(n) = A000040(A000720(A007908(n)-1)).
%e A276200 a(5) = 12343, because this is the largest prime less than 12345.
%p A276200 tcat:= (a,b) -> a*10^(1+ilog10(b))+b:
%p A276200 t:= 1: R:= NULL:
%p A276200 for i from 2 to 20 do
%p A276200 t:= tcat(t,i);
%p A276200 R:= R,prevprime(t);
%p A276200 od:
%p A276200 R; # _Robert Israel_, Oct 29 2024
%t A276200 Table[NextPrime[FromDigits[Flatten[IntegerDigits[Range[n]]]], -1], {n, 2, 19}]
%Y A276200 Cf. A000040, A000720, A007908, A074365, A151799.
%K A276200 nonn,base
%O A276200 2,1
%A A276200 _Ilya Gutkovskiy_, Aug 24 2016