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MathGroup Archive 2004

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Re: Re: Log[4]==2*Log[2]

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50609] Re: [mg50557] Re: [mg50520] Log[4]==2*Log[2]
  • From: Andrzej Kozlowski <andrzej at akikoz.net>
  • Date: Sun, 12 Sep 2004 04:42:24 -0400 (EDT)
  • References: <200409090917.FAA19334@smc.vnet.net> <200409100805.EAA04777@smc.vnet.net> <opsd4ehiyriz9bcq@monster.cox-internet.com> <2E4E6EFA-03F0-11D9-99E9-000A95B4967A@akikoz.net> <opsd59t8ufiz9bcq@monster.cox-internet.com>
  • Sender: owner-wri-mathgroup at wolfram.com

Yes, I agree. Indeed, what it does it tries to settle "exact equality" 
using inexact numbers, and it of course it can't be done. However,  as 
you correctly point out, had the quantities entered not been equal 
Mathematica would have settled the question. This is of course the 
reason why it does anything at all except just simply returning the 
input.

Andrzej


On 12 Sep 2004, at 02:07, DrBob wrote:

> *This message was transferred with a trial version of CommuniGate(tm) 
> Pro*
>>> Mathematica could not determine anything because it tries to
>>> compare the numbers "numerically" without using approximate numerical
>>> values, which can't be done.
>
> I agree it can't be done; so why is Mathematica trying to do that? 
> Documentation says:
>
>>> For exact numeric quantities, Equal internally uses numerical 
>>> approximations to establish inequality.
>
> Inequality usually can be established this way, but equality never can 
> be. I suppose that's what the error message "intends to mean". Equal 
> tried to establish INEQUALITY and failed, yet is unwilling to declare 
> EQUALITY.
>
> Hence the test could never be True for exact quantities. But...
>
> Log[2]==Log[2]
>
> True
>
> Sigh....
>
> Bobby
>
> On Sat, 11 Sep 2004 21:43:28 +0900, Andrzej Kozlowski 
> <andrzej at akikoz.net> wrote:
>
>> *This message was transferred with a trial version of CommuniGate(tm) 
>> Pro*
>> Actually, I don't think Mathematica does any real "determining" since
>> it does not replace the exact values given in the input by
>> numerical approximations. The message issued is, I think, purely
>> formal. Mathematica could not determine anything because it tries to
>> compare the numbers "numerically" without using approximate numerical
>> values, which can't be done. (You have to apply N for it to use
>> numerical values). That't what I meant by "not surprisingly". I don't
>> think I  really understand your point?
>>
>> ANdrzej
>>
>>
>> On 11 Sep 2004, at 01:52, DrBob wrote:
>>
>>> *This message was transferred with a trial version of CommuniGate(tm)
>>> Pro*
>>>>> Mathematica does not apply any simplification rules but justtries 
>>>>> to
>>>>> evaluate the expression numerically and, not
>>>>> surprisingly, it can't determine if the LHS is zero or not
>>>>> up to the required precision.
>>>
>>> On the contrary, I think the error message itself clearly indicates
>>> the difference IS zero to "the required precision". If 50 digits 
>>> extra
>>> precision isn't enough to determine that the difference ISN'T zero,
>>> why doesn't Equal return True?
>>>
>>> Bobby
>>>
>>> On Fri, 10 Sep 2004 04:05:56 -0400 (EDT), Andrzej Kozlowski
>>> <andrzej at akikoz.net> wrote:
>>>
>>>> On 9 Sep 2004, at 18:17, Andreas Stahel wrote:
>>>>
>>>>>
>>>>> To whom it may concern
>>>>>
>>>>> the following answer of Mathematica 5.0 puzzeled me
>>>>>
>>>>> Log[4]==2*Log[2]
>>>>> leads to
>>>>>
>>>>> N::meprec: Internal precision limit $MaxExtraPrecision = 50.` 
>>>>> reached
>>>>> while \
>>>>> evaluating -2\Log[2]+Log[4]
>>>>>
>>>>> with the inputs given as answer. But the input
>>>>>
>>>>> Log[4.0]==2*Log[2]
>>>>>
>>>>> leads to a sound "True"
>>>>>
>>>>> Simplify[Log[4]-2*Log[2]]
>>>>> leads to the correct 0, but
>>>>> Simplify[Log[4]-2*Log[2]==0]
>>>>> yields no result
>>>>>
>>>>> There must be some systematic behind thid surprising behaviour.
>>>>> Could somebody give me a hint please
>>>>>
>>>>> With best regards
>>>>>
>>>>> Andreas
>>>>> --
>>>>> Andreas Stahel       E-Mail: Andreas.Stahel at [ANTI-SPAM]hti.bfh.ch
>>>>> Mathematics, HTI     Phone: ++41 +32 32 16 258
>>>>> Quellgasse 21        Fax:   ++41 +32 321 500
>>>>> CH-2501 Biel         WWW:   www.hta-bi.bfh.ch/~sha
>>>>> Switzerland
>>>>>
>>>>>
>>>>
>>>> When you enter
>>>>
>>>> Log[4] - 2*Log[2] == 0
>>>>
>>>> Mathematica does not apply any simplification rules but just tries 
>>>> to
>>>> evaluate the expression numerically and, not surprisingly, it can't
>>>> determine if the LHS is zero or not up to the required precision.
>>>>
>>>> If you use
>>>>
>>>> Simplify[Log[4] - 2*Log[2] == 0]
>>>>
>>>> Mathematica first tries to evaluate the argument of Simplify and the
>>>> same thig happens as above, but then it actually applies Simplify to
>>>> the output and gets the right answer True.
>>>>
>>>> The best thing to do is:
>>>>
>>>>
>>>> Simplify[Unevaluated[Log[4]-2*Log[2]==0]]
>>>>
>>>>
>>>> True
>>>>
>>>> which avoids evaluation of the argument and instead uses Simplify on
>>>> the unevaluated input.
>>>>
>>>>
>>>>
>>>> Andrzej Kozlowski
>>>> Chiba, Japan
>>>> http://www.akikoz.net/~andrzej/
>>>> http://www.mimuw.edu.pl/~akoz/
>>>>
>>>>
>>>>
>>>
>>>
>>>
>>> --
>>> DrBob at bigfoot.com
>>> www.eclecticdreams.net
>>>
>>
>>
>>
>
>
>
> -- 
> DrBob at bigfoot.com
> www.eclecticdreams.net
>


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