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 A138961 #17 May 26 2022 09:53:13
%S A138961 1,3,41,617,823,643,9721,14593,3803,14405693,10939223,4156374407,
%T A138961 2663693,5603770631,1221751714624799,287108811653770498027,
%U A138961 74103167823547,11843077531813991,726216405947772436185983423,769725127,18274551225153265813469
%N A138961 a(n) = largest prime divisor of A138957(n).
%C A138961 For smallest prime divisors see A138960.
%H A138961 Robert Price, <a href="/A138961/b138961.txt">Table of n, a(n) for n = 1..60</a>
%t A138961 a = {}; b = {}; Do[w = RealDigits[n]; w = First[w]; Do[AppendTo[a, w[[Length[w] - k + 1]]], {k, 1, Length[w]}]; p = FromDigits[a]; AppendTo[b, First[Last[FactorInteger[p]]]], {n, 1, 18}]; b
%t A138961 A137957[n_] := FromDigits[Flatten[Reverse /@ IntegerDigits[Range[n]]]];
%t A138961 Table[First[Last[FactorInteger[A137957[n]]]], {n, 39}] (* _Robert Price_, May 10 2019 *)
%Y A138961 Cf. A104759, A000422, A116504, A007908, A116505, A104759, A138789, A138790, A138960.
%K A138961 nonn,base
%O A138961 1,2
%A A138961 _Artur Jasinski_, Apr 04 2008