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A046430 All the prime factors of repunit a(n) end with the digit 1. 1
1, 2, 4, 5, 10, 19, 20, 23, 25, 38, 50, 59, 76, 95, 100, 115, 125, 190, 250, 295, 317 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..21.

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

STATUS

approved

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Last modified July 30 18:40 EDT 2021. Contains 346359 sequences. (Running on oeis4.)