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A051626 Length of the period of decimal representation of 1/n, or 0 if 1/n terminates. 23
0, 0, 1, 0, 0, 1, 6, 0, 1, 0, 2, 1, 6, 6, 1, 0, 16, 1, 18, 0, 6, 2, 22, 1, 0, 6, 3, 6, 28, 1, 15, 0, 2, 16, 6, 1, 3, 18, 6, 0, 5, 6, 21, 2, 1, 22, 46, 1, 42, 0, 16, 6, 13, 3, 2, 6, 18, 28, 58, 1, 60, 15, 6, 0, 6, 2, 33, 16, 22, 6, 35, 1, 8, 3, 1, 18, 6, 6, 13, 0, 9, 5, 41, 6, 16, 21, 28, 2, 44, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,7

COMMENTS

For any prime number p: if a(p)>0, a(p) divides p-1. - David Spitzer, Jan 09 2017

LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000

Project Euler, Reciprocal cycles: Problem 26

Eric Weisstein's World of Mathematics, Repeating Decimal

Index entries for sequences related to decimal expansion of 1/n

FORMULA

a(n)=A132726(n,1); a(n)=a(A132740(n)); a(A132741(n))=a(A003592(n))=0. - Reinhard Zumkeller, Aug 27 2007

EXAMPLE

From M. F. Hasler, Dec 14 2015: (Start)

a(1) = a(2) = 0 because 1/1 = 1 and 1/2 = 0.5 have a finite decimal expansion.

a(3) = a(6) = a(9) = a(12) = 1 because 1/3 = 0.{3}*, 1/6 = 0.1{6}*, 1/9 = 0.{1}*, 1/12 = 0.08{3}* where the sequence of digits {...}* which repeats indefinitely is of length 1

a(7) = 6 because 1/7 = 0.{142857}* with a period {142857} of length 6.

a(17) = 16 because 1/17 = 0.{0588235294117647}* with a period of length 16.

a(19) = 18 because 1/19 = 0.{052631578947368421}* with a period {052631578947368421} of length 18. (End)

MAPLE

A051626 := proc(n) local lpow, mpow ;

    if isA003592(n) then

       RETURN(0) ;

    else

       lpow:=1 ;

       while true do

          for mpow from lpow-1 to 0 by -1 do

              if (10^lpow-10^mpow) mod n =0 then

                 RETURN(lpow-mpow) ;

              fi ;

          od ;

          lpow := lpow+1 ;

       od ;

    fi ;

end: # R. J. Mathar, Oct 19 2006

MATHEMATICA

r[x_]:=RealDigits[1/x]; w[x_]:=First[r[x]]; f[x_]:=First[w[x]]; l[x_]:=Last[w[x]]; z[x_]:=Last[r[x]];

d[x_] := Which[IntegerQ[l[x]], 0, IntegerQ[f[x]]==False, Length[f[x]], True, Length[l[x]]]; Table[d[i], {i, 1, 90}] (* Hans Havermann, Oct 19 2006 *)

fd[n_] := Block[{q}, q = Last[First[RealDigits[1/n]]]; If[IntegerQ[q], q = {}]; Length[q]]; Table[fd[n], {n, 100}] (* Ray Chandler, Dec 06 2006 *)

Table[Length[RealDigits[1/n][[1, -1]]], {n, 90}] (* Harvey P. Dale, Jul 03 2011 *)

PROG

(PARI) A051626(n)=if(1<n/=5^valuation(n, 5)<<valuation(n, 2), znorder(Mod(10, n)), 0) \\ M. F. Hasler, Dec 14 2015

(Python)

def A051626(n):

    if isA003592(n):

        return 0

    else:

        lpow=1

        while True:

            for mpow in range(lpow-1, -1, -1):

                if (10**lpow-10**mpow) % n == 0:

                    return lpow-mpow

            lpow += 1 # Kenneth Myers, May 06 2016

CROSSREFS

Essentially same as A007732. Cf. A002371, A048595, A006883, A036275, A114205, A114206.

Sequence in context: A137378 A293071 A084680 * A264808 A200229 A137785

Adjacent sequences:  A051623 A051624 A051625 * A051627 A051628 A051629

KEYWORD

nonn,base,easy,nice

AUTHOR

J. Lowell

EXTENSIONS

More terms from James A. Sellers

STATUS

approved

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Last modified November 18 05:25 EST 2017. Contains 294853 sequences.