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 2017-10-02, 02:39 #1 a1call     "Rashid Naimi" Oct 2015 Remote to Here/There 2·1,087 Posts Repeating Digits of Pi There is this thread in another forum. I am interested in members of this board's input. Thanks in advance. https://forum.cosmoquest.org/forumdi...and-Technology ETA: I made correction to my post on the other board: 11 changed to 1010. Last fiddled with by a1call on 2017-10-02 at 03:08
 2017-10-03, 17:13 #2 a1call     "Rashid Naimi" Oct 2015 Remote to Here/There 2·1,087 Posts The conversation has taken an interesting turn. If an ideal random number generator is guaranteed to output any given number, given infinite trials, can we conclude that any given finite sequence of digits will occur somewhere in the pi's decimal expansion?
2017-10-03, 20:27   #3
R. Gerbicz

"Robert Gerbicz"
Oct 2005
Hungary

27578 Posts

Quote:
 Originally Posted by a1call There is this thread in another forum. https://forum.cosmoquest.org/forumdi...and-Technology
Some part already solved in the "Does pi contain pi?" thread. Obviously we omit the trivial (identity) representation, and then it is impossible to find pi if you skip finitely many digits, because as it is pointed out pi would be rational.

The interesting (not solved there) part is that: can you get x if you delete infinitely many digits of a general x number?
If x is rational then the answer is yes (Why?).
If x is irrational (like pi) then you can still get x: let H={d: 0<=d<10 and d appears in x infinitely many times}. It is clear that there exists k, that after the k-th decimal digit of x you see only digits from H. The strategy is: keep the first k digits of x, then skip a digit, you can still find all digits (in order) of x, because these digits are in H, and hence these appears infinitely many times in x. After you have found the (k+1)-th digit at the t-th position then skip the (t+1)-th digit and use induction from here. Note that this is also a (harder) proof for the easier case, when x is rational, and ofcourse |H|>=2 if x is irrational (though it is not used).

Last fiddled with by R. Gerbicz on 2017-10-03 at 20:31 Reason: typo

 2017-10-05, 01:15 #4 a1call     "Rashid Naimi" Oct 2015 Remote to Here/There 41768 Posts As pointed out on the other board, I am having a lot of problems with the normal numbers: https://en.wikipedia.org/wiki/Normal_number Does anyone else see that the definition is based on the false assumption that a number with infinite digital expansion in base 10 will never have terminating digits in any other base or that it can't have a bias in the density of digits in some bases and not in others? Thanks in advance for any clarifications and/or inputs.
2017-10-05, 02:17   #5
science_man_88

"Forget I exist"
Jul 2009
Dumbassville

838410 Posts

Quote:
 Originally Posted by a1call As pointed out on the other board, I am having a lot of problems with the normal numbers: https://en.wikipedia.org/wiki/Normal_number Does anyone else see that the definition is based on the false assumption that a number with infinite digital expansion in base 10 will never have terminating digits in any other base or that it can't have a bias in the density of digits in some bases and not in others? Thanks in advance for any clarifications and/or inputs.
if it's non-repeating and non-terminating it would be irrational and hence not terminate in any rational base ... also your complaints show, you haven't read the wikipedia article in full where they declare things b-normal.

Last fiddled with by science_man_88 on 2017-10-05 at 02:28

2017-10-05, 02:41   #6
a1call

"Rashid Naimi"
Oct 2015
Remote to Here/There

2×1,087 Posts

Quote:
 Originally Posted by science_man_88 if it's non-repeating and non-terminating it would be irrational and hence not terminate in any rational base ... also your complaints show, you haven't read the wikipedia article in full where they declare things b-normal.
Quote:
 Intuitively this means that no digit, or (finite) combination of digits, occurs more frequently than any other, and this is true whether the number is written in base 10, binary, or any other base.
Rational base is your introduction. It is not present in the Wikipedia article.

2017-10-05, 02:43   #7
science_man_88

"Forget I exist"
Jul 2009
Dumbassville

838410 Posts

Quote:
 Originally Posted by a1call Rational base is your introduction. It is not present in the Wikipedia article.
well the integer bases are all rational so if you believe they are talking about intger bases the statement still stands.

 2017-10-05, 02:46 #8 a1call     "Rashid Naimi" Oct 2015 Remote to Here/There 2×1,087 Posts The phrase "integer base" does not exist in the article either.
2017-10-05, 04:20   #9
axn

Jun 2003

2·32·172 Posts

Quote:
 Originally Posted by a1call The phrase "integer base" does not exist in the article either.
Quote:
 Originally Posted by TFA In mathematics, a normal number is a real number whose infinite sequence of digits in every base b[1]
From the first line of wikipedia article, if you hover over the [1], you'll see this "The only bases considered here are natural numbers greater than 1"

 2017-10-05, 05:58 #10 a1call     "Rashid Naimi" Oct 2015 Remote to Here/There 2×1,087 Posts Thank you axn. Will check that out next time I am on a PC.
 2017-10-05, 15:58 #11 Dr Sardonicus     Feb 2017 Nowhere 2·3·857 Posts I don't know whether the number pi is normal to base ten (or to any other base). I default to the "assumption of ignorance" that it is normal, because I am ignorant of any reason to think it is not normal. (Also, the decimal digits calculated to date seem to pass the "randomness tests" thrown at them.) However, the "output of a random number generator" idea for the digits might come to grief, at least in hexadecimal. I might well be wrong here, but if the digits can reasonably be viewed as sequentially put out by a random number generator, then it seems to me there should be no way to calculate the nth digit without calculating all the previous digits first. And at least in hexadecimal, there is a method for finding the nth digit without calculating all the previous digits first.

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