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 A003021 M2007 #62 May 09 2025 23:10:14
%S A003021 2,11,101,13,137,9091,9901,909091,5882353,52579,27961,8779,99990001,
%T A003021 1058313049,121499449,9091,69857,21993833369,999999000001,
%U A003021 909090909090909091,5964848081,909091,1056689261,549797184491917
%N A003021 Largest prime factor of 10^n + 1.
%D A003021 J. Brillhart et al., Factorizations of b^n +- 1. Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 2nd edition, 1985; and later supplements.
%D A003021 Ehrhard Behrends, Five-Minute Mathematics, translated by David Kramer. American Mathematical Society (2008) p. 7.
%D A003021 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H A003021 <a href="/A003021/b003021.txt">Table of n, a(n) for n = 0..331</a>
%H A003021 J. Brillhart et al., <a href="http://dx.doi.org/10.1090/conm/022">Factorizations of b^n +- 1</a>, Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 3rd edition, 2002.
%H A003021 T. Granlund, <a href="http://gmplib.org/~tege/fac10p.txt">Factors of 10^n + 1</a>.
%H A003021 Makoto Kamada, <a href="https://stdkmd.net/nrr/repunit/10001.htm">Factorizations of 100...001</a>.
%H A003021 S. S. Wagstaff, Jr., <a href="http://www.cerias.purdue.edu/homes/ssw/cun/index.html">The Cunningham Project</a>.
%F A003021 a(n) = A006530(A062397(n)). - _Vincenzo Librandi_, Jul 12 2016
%t A003021 Table[FactorInteger[10^n + 1][[-1, 1]], {n, 0, 29}] (* _Alonso del Arte_, Oct 21 2011 *)
%o A003021 (PARI) for(n=0, 1e2, p=factor(10^n+1)[omega(10^n+1), 1]; print1(p, ", ")) \\ _Felix Fröhlich_, Aug 13 2014
%o A003021 (Magma) [Maximum(PrimeDivisors(10^n+1)): n in [0..40]]; // _Vincenzo Librandi_, Jul 12 2016
%Y A003021 Cf. A006530, A062397.
%K A003021 nonn,easy
%O A003021 0,1
%A A003021 _N. J. A. Sloane_
%E A003021 More terms from _Jason Earls_, Jul 11 2001
%E A003021 Terms up to a(280) in b-file from _D. S. McNeil_, Oct 22 2011
%E A003021 a(281)-a(310) in b-file from _Ray Chandler_, May 02 2017
%E A003021 a(311)-a(331) in b-file from _Max Alekseyev_, Apr 24 2019, Feb 13 2020, May 13 2022