A046430 All the prime factors of repunit a(n) end 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

1,2

Links

Table of n, a(n) for n=1..22.

P. De Geest, Repunits prime factors

Eric Weisstein's World of Mathematics, Repunit

Example

a(n)=10 -> 11*41*271*9091 ->1(1)*4(1)*27(1)*909(1).

Mathematica

Select[Range[60], Length[fi=FactorInteger[(10^#-1)/9]]==Length[Select[First/@fi, Mod[#, 10]==1&]] &](* Ray Chandler, Apr 24 2017 *)

Crossrefs

Cf. A000042, A004022, A046053.

Sequence in context: A049332 A335206 A096570 * A133040 A018424 A240100

Adjacent sequences: A046427 A046428 A046429 * A046431 A046432 A046433

Keyword

nonn,more,base

Author

Patrick De Geest, Jul 15 1998

Extensions

a(16)-a(21) from Ray Chandler, Apr 24 2017

a(22) from Max Alekseyev, May 14 2022

Status

approved