A112505 - OEIS

A112505

Number of primitive prime factors of 10^n-1.

1, 1, 1, 1, 2, 2, 2, 2, 1, 1, 2, 1, 3, 1, 2, 2, 2, 2, 1, 2, 3, 3, 1, 1, 3, 2, 2, 3, 5, 3, 3, 5, 2, 3, 3, 1, 3, 1, 1, 2, 4, 3, 4, 3, 2, 4, 2, 1, 2, 3, 4, 2, 4, 2, 4, 2, 3, 2, 2, 3, 7, 1, 5, 4, 2, 2, 3, 3, 3, 2, 2, 3, 3, 3, 3, 2, 4, 4, 6, 2, 5, 2, 3, 2, 3, 3, 3, 2, 5, 3, 7, 3, 1, 3, 5, 4, 3, 2, 4, 4

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OFFSET

1,5

COMMENTS

Also the number of primes whose reciprocal is a repeating decimal of length n. The number of numbers in each row of table A046107.

By Zsigmondy's theorem, a(n) >= 1. When a(n)=1, the corresponding prime is called a unique prime (see A007498, A040017 and A051627).

LINKS

Table of n, a(n) for n = 1..352

Eric Weisstein's World of Mathematics, Primitive Prime Factor

Eric Weisstein's World of Mathematics, Zsigmondy Theorem

Eric Weisstein's World of Mathematics, Unique Prime

MATHEMATICA

pp={}; Table[f=Transpose[FactorInteger[10^n-1]][[1]]; p=Complement[f, pp]; pp=Union[pp, p]; Length[p], {n, 66}]

CROSSREFS

Cf. A007138 (smallest primitive prime factor of 10^n-1), A102347 (number of distinct prime factors of 10^n-1), A046107.

Sequence in context: A368752 A323827 A275850 * A104638 A270648 A331034

Adjacent sequences: A112502 A112503 A112504 * A112506 A112507 A112508

KEYWORD

hard,nonn

AUTHOR

T. D. Noe, Sep 08 2005

EXTENSIONS

Terms to a(276) in b-file from T. D. Noe, Jun 01 2010

a(277)-a(322) in b-file from Ray Chandler, May 01 2017

a(323)-a(352) in b-file from Max Alekseyev, Apr 28 2022

STATUS

approved