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 A074836 #19 Jan 30 2025 21:03:11
%S A074836 1,53,353,2160617,479,146053847,18533465459,18588601,444444443,
%T A074836 10987654321234567891,41824630287288128897,24899126702236725259,
%U A074836 131211109876543212345678910111213,59722151033999393,1081735780698166140181,40378532802777469135803086419727527803285379
%N A074836 a(1) = 1, a(n) = the largest prime divisor of concatenation of sequence (n,n-1,..,2,1,2,..,n-1,n) A007942.
%H A074836 Sean A. Irvine, <a href="/A074836/b074836.txt">Table of n, a(n) for n = 1..55</a>
%H A074836 M. Fleuren, <a href="http://www.gallup.unm.edu/~smarandache/SmMirror.txt">Smarandache Mirror Sequence</a>.
%e A074836 a(4) is 2160617 because 4321234 = 2.'2160617'.
%t A074836 Table[FactorInteger[FromDigits[Join[Flatten[IntegerDigits/@ Range[ n,1,-1]],Flatten[IntegerDigits/@Range[2,n]]]]][[-1,1]],{n,2,15}] (* _Harvey P. Dale_, Sep 05 2015 *)
%Y A074836 Cf. A007942.
%K A074836 nonn,base
%O A074836 1,2
%A A074836 _Jason Earls_, Sep 09 2002
%E A074836 Edited by _Charles R Greathouse IV_, Apr 28 2010
%E A074836 More terms from _Harvey P. Dale_, Sep 05 2015
%E A074836 a(1)=1 inserted by _Sean A. Irvine_, Jan 30 2025