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 A109840 #5 Sep 02 2023 15:29:23
%S A109840 31,59,443,13933,2609,83089,18937669,57697,71615663503,
%T A109840 161768817795401,4356390089,4668342004597,109230695824219,
%U A109840 2770122504473483,217596035578054625221,1721572826917,659160492491
%N A109840 Largest prime factor of the reverse concatenation of the first n consecutive odd numbers.
%e A109840 n = 6: a(6) = 83089 because 131197531 = 1579 * 83089.
%t A109840 s=1;ss={};Do[news=FromDigits[Flatten[{IntegerDigits[n], IntegerDigits[s]}]];fi=FactorInteger[news][[ -1, 1]];s=news;AppendTo[ss, fi], {n, 3, 60, 2}]; A109840=ss
%t A109840 With[{nn=17},Table[FactorInteger[FromDigits[Flatten[IntegerDigits/@Reverse[Range[1,2n+1,2]]]]][[-1,1]],{n,nn}]] (* _Harvey P. Dale_, Sep 02 2023 *)
%Y A109840 Cf. A109837 = smallest prime factor of the reverse concatenation of the first n consecutive odd numbers.
%K A109840 base,nonn
%O A109840 1,1
%A A109840 _Zak Seidov_, Jul 05 2005