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 A075022 #23 May 26 2022 09:52:05
%S A075022 1,3,41,617,823,643,9721,14593,3803,1234567891,630803,2110805449,
%T A075022 869211457,205761315168520219,8230452606740808761,1231026625769,
%U A075022 584538396786764503,801309546900123763,833929457045867563
%N A075022 a(1) = 1; for n>1, a(n) = the largest prime divisor of the number C(n) formed from the concatenation of 1,2,3,... up to n.
%H A075022 N. J. A. Sloane, <a href="/A075022/b075022.txt">Table of n, a(n) for n = 1..56</a> [First 50 terms from Zak Seidov]
%e A075022 a(4) = 617 since 1234 = 2*617.
%t A075022 a = {}; b = {}; Do[w = RealDigits[n]; w = First[w]; Do[AppendTo[a, w[[k]]], {k, 1, Length[w]}]; p = FromDigits[a]; AppendTo[b, First[Last[FactorInteger[p]]]], {n, 1, 25}]; b (* _Artur Jasinski_, Apr 04 2008 *)
%t A075022 Table[FactorInteger[FromDigits[Flatten[IntegerDigits/@Range[n]]]][[-1,1]],{n,20}] (* _Harvey P. Dale_, Aug 31 2015 *)
%Y A075022 Cf. A075019, A075020, A075021.
%Y A075022 Cf. A104759, A000422, A116504, A007908, A116505, A104759, A138789, A138790, A138960, A138961, A138962.
%K A075022 base,nonn
%O A075022 1,2
%A A075022 _Amarnath Murthy_, Sep 01 2002
%E A075022 More terms from _Sascha Kurz_, Jan 03 2003