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MathGroup Archive 2006

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Re: Re: Simplify UnitStep expressions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg69357] Re: [mg69326] Re: Simplify UnitStep expressions
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Sat, 9 Sep 2006 03:26:32 -0400 (EDT)

I find it very curious that, with "due respect", you seem to believe  
that you know better how Simplify and FullSimplify work than the  
person who actually wrote both of them. But of course this is not my  
problem.

Concerning your other post in this thread: it may be attractive to  
some to think in terms of what computer programs "know" and "do not  
know", but actually it is specific algorithms that are responsible  
for the output one gets. And the algorithm involved is CAD. (OF  
course you are free to think that Simplify "does not know" that b is  
positive while FullSimplify "knows it", but to me it seems  a  
somewhat naive and not very useful approach to such matters).

As for your example in this post, the correct syntax is:


In[1]:=
Assuming[a > 2, Simplify[Not[a > 2] ]]

Out[1]=
False

Andrzej Kozlowski


On 7 Sep 2006, at 17:30, p-valko at tamu.edu wrote:

> With all due respect I am afraid the phenomenon has little to do with
> the actual algorithms used in Simplify[] and FullSimplify[], but  
> rather
> with the way how assumptions are treated in general.
>
> To illustrate my statement, I show an example without any Simplify 
> [] or
> FullSimplify[]:
>
> In:
> In[1]:=
> Assuming[a > 2, Simplify[Not[a > 2] ]]
>
> Out[1]=
> False
>
>
> I would like to get an answer False, but Mathematica gives
>
> Out:
> a <= 2
>
> Pretty surprising result ! ! !
>
> Regards
> Peter
>
> Adam Strzebonski wrote:
>> Andrzej Kozlowski wrote:
>>>
>>> On 5 Sep 2006, at 16:20, Adam Strzebonski wrote:
>>>
>>>> Andrzej Kozlowski wrote:
>>>>
>>>>> On 1 Sep 2006, at 11:41, L. Dwynn Lafleur wrote:
>>>>>
>>>>>> The following is transcribed from a Mathematica 5.2 notebook in
>>>>>> Windows XP:
>>>>>>
>>>>>> In[1]:= Simplify[UnitStep[a-x/b], a-x/b > 0]
>>>>>> Out[1]= 1
>>>>>>
>>>>>> In[2]:= Simplify[UnitStep[a-Pi/b], a-Pi/b > 0]
>>>>>> Out[2]= UnitStep[a-Pi/b]
>>>>>>
>>>>>> Why does the second output different from the first?  I know  
>>>>>> it has
>>>>>> something to do with the fact that Pi is internally defined in
>>>>>> Mathematica
>>>>>> because a similar result occurs Pi is replaced with E, but what
>>>>>> logic is
>>>>>> being followed?
>>>>>>
>>>>>> --
>>>>>> ======================================
>>>>>>  L. Dwynn Lafleur
>>>>>>  Professor of Physics
>>>>>>  University of Louisiana at Lafayette
>>>>>>  lafleur at louisiana.edu
>>>>>> ======================================
>>>>>>
>>>>> Curiously, if you use FullSimplify rather then Simplify you  
>>>>> will get:
>>>>> FullSimplify[UnitStep[a-Pi/b], a-Pi/b > 0]
>>>>> 1
>>>>> The same holds if Pi is replaces by E, or indeed by explicit
>>>>> functions of E or Pi such as Pi^2, E^Pi etc. In all such cases
>>>>> FullSimplify works but Simplify does not work. Strange.
>>>>> Andrzej Kozlowski
>>>>
>>>>
>>>> The cylindrical algebraic decomposition (CAD) algorithm used by   
>>>> Simplify
>>>> to prove inference requires polynomial inequalities with  
>>>> rational  number
>>>> coefficients. a-x/b > 0 is equivalent to a polynomial inequality
>>>> -(a*b^2) + b*x < 0 which has rational number coefficients.
>>>> a-Pi/b > 0 is equivalent to a polynomial inequality -(a*b^2) +  
>>>> b*Pi  < 0
>>>> which has a numeric coefficient Pi which is not a rational number.
>>>>
>>>> Mathematica has two ways of dealing with nonrational numeric
>>>> coefficients in CAD. One is to replace each nonrational coefficient
>>>> with a new variable. This method always allows to decide inference
>>>> (modulo the ability to zero-test the exact numeric constants), but
>>>> it is potentially very expensive - CAD has a doubly exponential
>>>> complexity in the number of variables and we add a new variable for
>>>> each nonrational coefficient. The second method replaces  
>>>> nonrational
>>>> numeric coefficients with their approximations. This is much less
>>>> expensive, but in some cases it fails to decide inference.
>>>> Simplify uses the second method which in this case is insufficient.
>>>>
>>>> FullSimplify uses more transformations, and one of the additional
>>>> transformations succeeds.
>>>>
>>>> Best Regards,
>>>>
>>>> Adam Strzebonski
>>>> Wolfram Research
>>>>
>>>>
>>>>
>>>
>>> Thanks for the explanation. I feel I should have guessed it, but   
>>> there
>>> is one thing that still puzzles me. What exactly makes the  rational
>>> approximation fail for Pi or E, since it seems to work fine  for
>>> algebraic numbers such as Sqrt[2] or 3^(1/3)? It certainly can't  be
>>> anything to do with Pi or E not being algebraic, so presumably  
>>> it  is
>>> something to do with the way the rational approximation is  
>>> chosen?  This
>>> sounds very interesting; could you explain the exact reason why  the
>>> rational approximation in this case doe snot work and in what  other
>>> cases will it not work in general? It sounds like the reason   
>>> might be
>>> mathematically interesting (?).
>>>
>>> Andrzej Kozlowski
>>>
>>
>> For Pi or E the assumption mechanism uses inexact (bignum)
>> approximations and constructs CAD with inexact sample point
>> coordinates. If we have an inequality f(X)<0 and we do not
>> find a cell with a sample point P for which f(P)<0, but we
>> do find a cell with a sample point P for which f(P) is
>> a bignum zero (for instance 0``20) then we cannot tell
>> whether f(X)<0 has any solutions or not.
>>
>> For algebraic numbers the assumption mechanism does not use
>> approximations. It replaces the algebraic numbers with new
>> variables, because in this case it does not contribute that
>> much to the complexity. We do not need to compute projections
>> wrt. the new variables. Instead we make the variables last in
>> the projection ordering, and in the lifting phase we only lift
>> the one-point cell which corresponds to the new variables being
>> equal to the corresponding algebraic numbers.
>>
>> Best Regards,
>>
>> Adam Strzebonski
>> Wolfram Research
>


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