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MathGroup Archive 2006

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Re: comparing implicit 0 with machine floats

  • To: mathgroup at smc.vnet.net
  • Subject: [mg71211] Re: [mg71108] comparing implicit 0 with machine floats
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Fri, 10 Nov 2006 06:38:22 -0500 (EST)
  • References: <11095316.1163039426494.JavaMail.root@eastrmwml01.mgt.cox.net> <F3C6DEC6-64C4-45D4-98CC-8A79F52F3B0A@mimuw.edu.pl> <445AE1FB-31B2-4C25-841A-6F03BFE2FC68@mimuw.edu.pl>

I forgot that one has to be careful using Timing in the way I did  
below in Mathematica. What I should have done was:

In[1]:=
Timing[Simplify[Sqrt[2] + Sqrt[3] -
      Sqrt[5 + 2*Sqrt[6]] == 0.]]

Out[1]=
{0.046394999999999964*Second, True}

In[2]:=
Developer`ClearCache[]

In[3]:=
Timing[Simplify[Sqrt[2] + Sqrt[3] -
     Sqrt[5 + 2*Sqrt[6]]]]

Out[3]=
{0.03863100000000008*Second,
   Sqrt[2] + Sqrt[3] -
    Sqrt[5 + 2*Sqrt[6]]}

There still seems to be a difference in Timing but a much smaller  
one. Clearly Simplify performs some numerical zero testing.  I tried  
to control it using Developer`SetSystemOptions   and the option  
AlgebraicZeroTestMaxAccuracy in SimplificationOptions, but changing  
its settings does not seem to make any difference.

Andrzej Kozlowski



On 9 Nov 2006, at 18:39, Andrzej Kozlowski wrote:

> In fact, things look even more surprising (to me anyway).
>
> In[1]:=
> Simplify[Sqrt[2] + Sqrt[3] - Sqrt[5 + 2*Sqrt[6]] == 0.]//Timing
>
> Out[1]=
> {0.046486 Second,True}
>
> while
>
> In[2]:=
> Timing[Simplify[Sqrt[2] + Sqrt[3] - Sqrt[5 + 2*Sqrt[6]]]]
>
> Out[2]=
> {0.0008739999999998194*Second, Sqrt[2] + Sqrt[3] -
>    Sqrt[5 + 2*Sqrt[6]]}
>
> Note that the first evaluation took a non-negligible amount of  
> time. One reason why I am surprised is that I would have thought  
> one would need to increase $MaxExtraPrecision to something like
>
> Accuracy[0.]
>
> 307.653
>
> Andrzej Kozlowski
>
>
>
> On 9 Nov 2006, at 12:10, Andrzej Kozlowski wrote:
>
>> Yes indeed, it does. This is rather curious because Simplify does  
>> not actually reduce Sqrt[2] + Sqrt[3] - Sqrt[5 + 2*Sqrt[6]] to 0  
>> (since Simplify does not make use of RootReduce) one needs  
>> FullSimplify to do that.
>>
>> In[51]:=
>> Simplify[Sqrt[2] + Sqrt[3] - Sqrt[5 + 2*Sqrt[6]]]
>>
>> Out[51]=
>> Sqrt[2] + Sqrt[3] - Sqrt[5 + 2*Sqrt[6]]
>>
>> In[52]:=
>> FullSimplify[Sqrt[2] + Sqrt[3] - Sqrt[5 + 2*Sqrt[6]]]
>>
>> Out[52]=
>> 0
>>
>> Andrzej Kozlowski
>>
>> On 9 Nov 2006, at 11:30, Bob Hanlon wrote:
>>
>>> Using Simplify will force the comparison
>>>
>>> Sqrt[2] + Sqrt[3] - Sqrt[5 + 2*Sqrt[6]] == 2.//Simplify
>>>
>>> False
>>>
>>> Sqrt[2] + Sqrt[3] - Sqrt[5 + 2*Sqrt[6]] == N[10^3]//Simplify
>>>
>>> False
>>>
>>>
>>> Bob Hanlon
>>>
>>> ---- Andrzej Kozlowski <akoz at mimuw.edu.pl> wrote:
>>>> Consider the following:
>>>>
>>>>
>>>> Sqrt[2] + Sqrt[3] - Sqrt[4 + 2*Sqrt[6]] == 2.
>>>>
>>>> False
>>>>
>>>> No problem here. Now let's make a small change
>>>>
>>>> In[17]:=
>>>> Sqrt[2] + Sqrt[3] - Sqrt[5 + 2*Sqrt[6]] == 2.
>>>>
>>>> Out[17]=
>>>>
>>>> Sqrt[2] + Sqrt[3] - Sqrt[5 + 2*Sqrt[6]] == 2.
>>>>
>>>> In fact the expression on the LHS  is exactly 0:
>>>>
>>>> In[19]:=
>>>> RootReduce[Sqrt[2]+Sqrt[3]-Sqrt[5+2*Sqrt[6]]]
>>>>
>>>> Out[19]=
>>>>
>>>> The curious thing is that if you try a comparison between a zero of
>>>> this kind and any machine float, however large, Mathematica 5.1  
>>>> will
>>>> return the original input:
>>>>
>>>> In[20]:=
>>>> Sqrt[2] + Sqrt[3] - Sqrt[5 + 2*Sqrt[6]] == N[10^3]
>>>>
>>>> Out[20]=
>>>> Sqrt[2] + Sqrt[3] - Sqrt[5 + 2*Sqrt[6]] == 1000.
>>>>
>>>> yet if the number of the left hand side is altered, however  
>>>> slightly,
>>>> the comparison will be made:
>>>>
>>>>
>>>> Sqrt[2] + Sqrt[3] - Sqrt[5 + 2*Sqrt[6+1/10^20]] == 2.
>>>>
>>>> False
>>>>
>>>> This suggests that Mathematica actually did perform a  
>>>> computation of
>>>> the left hand side in the examples where it just returned the input
>>>> and having discovered that it could not determine if the LHS is an
>>>> exact zero decided "not to answer the question". But this seems  
>>>> quite
>>>> unreasonable; after all it is not being asked if the LHS is an  
>>>> exact
>>>> 0, or even an approximate 0, but if it is an approximate large  
>>>> number
>>>> like 1000., and this it certainly can decide.
>>>>
>>>> I believe this used to be handled differently (better?) in older
>>>> versions of Mathematica but I no longer have any installed to  
>>>> check.
>>>>
>>>> Andrzej Kozlowski
>>>>
>>>
>>
>


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