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 A046430 #22 Feb 16 2025 08:32:39 %S A046430 1,2,4,5,10,19,20,23,25,38,50,59,76,95,100,115,125,190,250,295,317,380 %N A046430 Lengths of repunits with all prime factors ending with the digit 1. %H A046430 P. De Geest, <a href="https://www.worldofnumbers.com/repunits.htm">Repunits prime factors</a> %H A046430 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Repunit.html">Repunit</a> %e A046430 a(n) = 10 -> 11*41*271*9091 -> 1(1)*4(1)*27(1)*909(1). %t A046430 Select[Range[60],Length[fi=FactorInteger[(10^#-1)/9]]==Length[Select[First/@fi,Mod[#,10]==1&]] &](* _Ray Chandler_, Apr 24 2017 *) %Y A046430 Cf. A000042, A004022, A046053. %K A046430 nonn,more,base %O A046430 1,2 %A A046430 _Patrick De Geest_, Jul 15 1998 %E A046430 a(16)-a(21) from _Ray Chandler_, Apr 24 2017 %E A046430 a(22) from _Max Alekseyev_, May 14 2022 %E A046430 Definition corrected by _Max Alekseyev_, Aug 19 2024