login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 59th year, we have over 358,000 sequences, and we’ve crossed 10,300 citations (which often say “discovered thanks to the OEIS”).

Other ways to Give
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A081318 Integers n such that the reciprocal of the largest prime factor of 10^n-1 is not a repeating decimal fraction with a period of n. 2
6, 18, 22, 30, 32, 38, 42, 46, 54, 66, 74, 78, 82, 90, 94, 96, 110, 118, 132, 138, 146, 154, 162, 174, 186, 194, 198, 206, 210, 218, 228, 231, 240, 242, 254, 258, 260, 264, 266, 268, 274, 282, 284, 286, 298, 300, 306, 310, 318, 322, 334, 338, 344, 348 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For all but three of the terms through a(41)=274, the reciprocal of the largest prime factor of 10^a(n)-1 is a decimal fraction with a period of a(n)/2. Of the three exceptions, there are two (a(32)=231 and a(38)=264) where the period is a(n)/3, and one (a(19)=132) where the period is a(n)/4. - Jon E. Schoenfield, Jun 27 2010

LINKS

Table of n, a(n) for n=1..54.

Makoto Kamada, Factorizations of 11...11 (repunit).

FORMULA

Integers n such that A061075(n) is not equal to A005422(n).

EXAMPLE

30 is in the sequence because the factorization of 10^30-1 is 3^3*7*11*13*31*37*41*211*241*271*2161*9091*2906161 and 2906161 occurs already in 10^15-1=3^3*31*37*41*271*2906161 producing a decimal fraction with a period of 15, (1/2906161=0.000000344096559000000344096559000000344...)

CROSSREFS

Cf. A081317.

Sequence in context: A186889 A022418 A350848 * A275168 A236359 A011775

Adjacent sequences: A081315 A081316 A081317 * A081319 A081320 A081321

KEYWORD

more,nonn

AUTHOR

Hugo Pfoertner, Mar 18 2003

EXTENSIONS

More terms from Hans Havermann, May 31 2003

Terms a(31)-a(37) from Jon E. Schoenfield, Jun 19 2010

Terms a(38)-a(41) added, link added, and earlier comment expanded by Jon E. Schoenfield, Jun 27 2010

a(42)-a(54) from Max Alekseyev, Aug 17 2013, Apr 26 2022

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified November 27 19:48 EST 2022. Contains 358406 sequences. (Running on oeis4.)