# Repeating Decimals

1. Nov 28, 2007

### ebaywannabe

This is a simple question and I'd appreciate a simple answer. ;-)

So, 2/3 is .6666... Is it a repeating decimal like this only in base 10 decimal system? Is this just an artifact of using base 10? It's still rational, even though you can't write it out? What is the standard notation for this type of thing? 10/11 is another example. Is this just due to the base 10? If so, are there repeaters that repeat independently of the number system you are using? Or is this kind of "weird" behavior number system specific?

Thanks.

2. Nov 28, 2007

### robert Ihnot

What's 2/3 look like in base 3? Hint: What does 2 look like and what does 6 look like in base 3?

3. Nov 28, 2007

### D H

Staff Emeritus
If you count 0 as a repeating decimal (e.g., 1/2=0.5000...), then every rational number expressed as a decimal fraction is a repeating decimal. If you want to be picky, things like 1/2 are called terminal decimals. Now, what about other bases? Being picky again, the word "decimal" implies base 10. The generic term is "place-value notation". Every rational number has either a terminating or repeating place-value notation in any integer base. For example 1/2 in base three is 0.111... A number that does not have a terminating or repeating place-value notation in a particular integer base (e.g., pi in base 10) does not have a terminating or repeating place-value notation in any integer base. The only numbers that behave this way are the irrationals, and all of the irrationals behave this way.

4. Nov 28, 2007

### Xevarion

So, here's an interesting question: what proportion of rational numbers in [0, 1] are repeating and what proportion are terminal in base $b$? Define the proportion of repeating rational numbers to be
$$\lim_{N\to \infty} \frac{\#\left\{\frac{p}{q} : q \le N, (p, q) = 1, \frac{p}{q} \text{ is repeating}\right\}}{\#\left\{\frac{p}{q} : q \le N, (p, q) = 1\right\}}.$$

5. Nov 28, 2007

### Office_Shredder

Staff Emeritus
Note that anything that terminates has two representations, e.g. 1/2=.5=.4999999... one of which isn't just trivial zeroes over and over again

6. Nov 28, 2007

### Xevarion

Perhaps we should be clear about our definitions then: a place-value representation is repeating iff there is a unique place-value representation, and that representation has nonzero terms after the $$n^{th}$$ place for any $$n$$. A representation is terminating iff there is some place-value representation which only has finitely many nonzero terms.

7. Nov 28, 2007

### CRGreathouse

The proportion is zero. Almost all denominators will have a prime factor not dividing the base.

8. Nov 28, 2007

### CRGreathouse

The denominator is http://www.research.att.com/~njas/sequences/A002088 [Broken](N) ~ $3N^2/\pi^2$. The numerator varies a lot, but I'm sure it's $o(N^2)$.

Last edited by a moderator: May 3, 2017
9. Nov 28, 2007

### Hurkyl

Staff Emeritus
What do you mean "you can't write it out?" You wrote it out in this very passage: '2/3'.

10. Nov 29, 2007

### mathwonk

look at the proof that a rational number is repeating. it has nothing to do with base 10.

11. Nov 30, 2007

### HallsofIvy

Where does that say anything about "repeating"? Wouldn't the decimal representation of $\pi$ fit that?

12. Nov 30, 2007

### Xevarion

haha I guess I wasn't sufficiently specific there. Yeah I meant to exclude irrationals, so my definition should also specify that after some $n$, the sequence of digits is periodic.

Another interesting question: how long can the period be, as a function of the size of the denominator? What is the average relationship?

13. Nov 30, 2007

### CRGreathouse

My answer was only considering rational numbers. The proportion of rational numbers that are terminating in base b (b fixed) is zero. _The proportion of real nunmbers that are terminating in base b is also zero, of course.)

14. Nov 30, 2007

### CRGreathouse

The length of the period of a/b (if a/b is in lowest terms) divides $\varphi(b)$, so the length of the period of a/b is between 1 and b - 1. I'm not aware of any results on the average length of periods.

Also, you can divide out any factors of the base in the denominator. In base 10, factors of 2 and 5 don't change the period.

15. Nov 30, 2007

### sathish mat

may i know please the meaning of squaring the circle

Last edited: Nov 30, 2007
16. Nov 30, 2007

### sathish mat

what is the meaning of locally exact in complex

17. Nov 30, 2007

### D H

Staff Emeritus
sathish,

First, welcome to Physics Forums.

Now a word of admonition: Please don't hijack existing threads to ask a question that is quite unrelated to the topic at hand. The thing to do is to start a new thread with your questions. These sound like homework questions, so I would suggest posting them in our homework section.

18. Dec 1, 2007

### mathwonk

thank you and may i ask the proof of the poincare conjecture?

19. Dec 6, 2007

### robert Ihnot

Originally Posted by Xevarion
Another interesting question: how long can the period be, as a function of the size of the denominator? What is the average relationship?

97 has a period of 96 decimals. Looking into the matter, 257 has a period of 256 = 2^8. These are called "full reptend primes."

William Shanks found pi to 707 decimals, but was only correct to 527 of them; also worked out tables of the length of all reciprocals of primes to 20,00. Proceeds of the Royal Society, London, #22, 1874.

C R Greathouse: The length of the period of a/b (if a/b is in lowest terms) divides , so the length of the period of a/b is between 1 and b - 1.

The length of the period is the smallest power n such that 10^n==1 Mod b. So the length of the period is b-1 when 10 is a primitive root of b. Otherwise it is a divisor of b-1.

For example, Sloane's Table A046146, http://mathworld.wolfram.com/PrimitiveRoot.html lists 3 as the smallest primitive root of 7. BUT 3^6 Mod 7 is the same as 10^6 Mod 7, so the length of the period is 6. Similarly we can see that if 3 is the smallest primitive root of 17, we have 3==3+17==20 Mod 17. Looking at (20)^8==(2^8)(10)^8==(1)*10^8==-1. So that 10^16 is the smallest power of 10 congruent to 1. Thus the period of 17 is 16.

Last edited: Dec 6, 2007