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A179192 Numbers n, not relatively prime to 10, such that the decimal form of the period of 1/n is prime. 1
12, 18, 30, 36, 45, 48, 75, 120, 180, 192, 198, 270, 288, 300, 330, 360, 450, 480, 495, 750, 768, 1152, 1200, 1584, 1800, 1875, 1920, 1980, 1998, 2304, 2700, 2880, 3000, 3072, 3300, 3330, 3600, 3690, 4500, 4800, 4950, 4995, 5625, 7500, 7680, 9090, 11520, 12000, 12288, 15840, 18000, 18432, 18750, 19200, 19800, 19980, 19998 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The sequence A175545 (numbers n such that the decimal form of the period of 1/n is prime) is only concerned with numbers n such that gcd(n,10)=1. Each number n such that gcd(n,10)<>1 generates a quotient where there exist a sequence of digits which is periodic after a finite sequence of digits, for example 1/36 = .0277777.... and 7 is periodic.

The prime numbers corresponding to this sequence are :

3, 5, 3, 7, 2, 3, 3, 3, 5, 3, 5, 37, 2, 3, 3, 7, 2, 3, 2,...

REFERENCES

H. Rademacher and O. Toeplitz, Von Zahlen und Figuren (Springer 1930, reprinted 1968), ch. 19, 'Die periodischen Dezimalbrueche'.

LINKS

Table of n, a(n) for n=1..57.

FORMULA

Union of A179192 and A175545 is A061564.

EXAMPLE

1584 is in the sequence because 1/1584 = .0006313131313131313131... and 31 is prime.

MATHEMATICA

Reap[Do[p=RealDigits[1/n][[1, -1]]; If[GCD[10, n]>1 && Head[p] === List, While[p[[-1]] == 0, p=Most[p]]; If[PrimeQ[FromDigits[p]], Sow[n]]], {n, 20000}]][[2, 1]]

CROSSREFS

Cf. A175557, A175555, A178505, A045572, A002329

Sequence in context: A071354 A006622 A124269 * A112054 A225576 A275082

Adjacent sequences:  A179189 A179190 A179191 * A179193 A179194 A179195

KEYWORD

nonn,base

AUTHOR

Michel Lagneau, Jul 01 2010

EXTENSIONS

Sequence corrected by T. D. Noe, Nov 18 2010

STATUS

approved

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Last modified November 18 14:15 EST 2017. Contains 294893 sequences.