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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. 1
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 (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..50.

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: A025163 A186889 A022418 * 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(50) from Max Alekseyev, Aug 17 2013

STATUS

approved

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Last modified September 16 07:19 EDT 2021. Contains 347469 sequences. (Running on oeis4.)