A046430 Lengths of repunits with all prime factors ending with the digit 1.
1, 2, 4, 5, 10, 19, 20, 23, 25, 38, 50, 59, 76, 95, 100, 115, 125, 190, 250, 295, 317, 380
Offset: 1
Examples
a(n) = 10 -> 11*41*271*9091 -> 1(1)*4(1)*27(1)*909(1).
Links
- P. De Geest, Repunits prime factors
- Eric Weisstein's World of Mathematics, Repunit
Programs
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Mathematica
Select[Range[60],Length[fi=FactorInteger[(10^#-1)/9]]==Length[Select[First/@fi,Mod[#,10]==1&]] &](* Ray Chandler, Apr 24 2017 *)
Extensions
a(16)-a(21) from Ray Chandler, Apr 24 2017
a(22) from Max Alekseyev, May 14 2022
Definition corrected by Max Alekseyev, Aug 19 2024