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Re: Re: Re: algebraic numbers

  • To: mathgroup at
  • Subject: [mg106246] Re: [mg106220] Re: [mg106192] Re: algebraic numbers
  • From: Andrzej Kozlowski <akoz at>
  • Date: Wed, 6 Jan 2010 05:59:02 -0500 (EST)
  • References: <> <hhpl0g$9l1$> <> <> <op.u52ai6jwtgfoz2@bobbys-imac.local>

Well, you are obviously misunderstanding  what I am trying to explain 
but I have no desire to spend any more time on it. I give up.

Perhaps you should try to explain yourself why Mathematica gives

In[1]:= Element[1.2, Rationals]

Out[1]= False

In[2]:= Element[1.2, Reals]

Out[2]= True

and you might also read

(but that's the last time I posting anything to do with any logic or 
mathematics here.)

Andrzej Kozlowski

On 5 Jan 2010, at 22:31, DrMajorBob wrote:

> RandomReal[] returns numbers from a countable set of rationals.
> Or call them reals, if you must; it still selects from a countable set 
of possibilities... not from the uncountable unit interval in the reals.
> The range of RandomReal[] is a set of measure zero, just like the 
algebraic numbers.
> Bobby
> On Tue, 05 Jan 2010 02:08:24 -0600, Andrzej Kozlowski 
<akoz at> wrote:
>> On 5 Jan 2010, at 15:47, DrMajorBob wrote:
>>> If computer reals are THE reals, why is it that RandomReal[{3,4}] 
>>> never return Pi, Sqrt[11], or ANY irrational?
>> It can't possibly do that because these are computable real numbers 
the set of computable real numbers if countable and has measure 0. 
Computable numbers can never be the outcome of any distribution that 
selects numbers randomly from a real interval.
>> The most common mistake people make about real numbers is to think 
that numbers such as Sqrt[2] or Pi as being in some sense typical 
examples of an irrational number or a transcendental number but they are 
not. They are very untypical because they are computable: that is, there 
exists a formula for computing as many of their digits as you like. But 
we can prove that the set of all reals with this property is countable 
and of measure 0. So Sqrt[2] is a very untypical irrational and Pi a 
very untypical transcendental. So what do typical real look like? Well, 
I think since a "typical" real is not computable we cannot know all of 
its digits and we cannot know any formula for computing them. But we can 
know a finite number of these digits. So this looks to me very much like 
the Mathematica concept of Real - you know a specified number of 
significant digits and you know that there are infinitely many more than 
you do not know. It seems to me the most natural way to think about 
non-computable reals.
>> Roger Penrose, by the way, is famous for arguing that our brain is 
somehow able to work with non-computable quantities, although of course 
not by using digital expansions. But this involves quantum physics and 
has been the object of a heated dispute since the appearance of "The 
emperor's New Mind".
> --
> DrMajorBob at

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