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MathGroup Archive 2007

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Re: question 1

  • To: mathgroup at smc.vnet.net
  • Subject: [mg75875] Re: [mg75810] question 1
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Sat, 12 May 2007 03:12:21 -0400 (EDT)
  • References: <200705110924.FAA05900@smc.vnet.net> <A52D6750-3017-4EF6-82F7-DFE4CB83812F@mimuw.edu.pl> <CB1CAE8E-3B30-4645-9279-14C936512446@mimuw.edu.pl> <E3CE072F-72C3-4B5C-B24D-DCFB113AA3B9@mimuw.edu.pl>

On 12 May 2007, at 11:18, Andrzej Kozlowski wrote:

>
> On 12 May 2007, at 10:37, Andrzej Kozlowski wrote:
>
>> *This message was transferred with a trial version of CommuniGate 
>> (tm) Pro*
>>
>> On 12 May 2007, at 10:23, Andrzej Kozlowski wrote:
>>
>>> *This message was transferred with a trial version of CommuniGate 
>>> (tm) Pro*
>>> On 11 May 2007, at 18:24, dimitris wrote:
>>>
>>>> I have
>>>>
>>>> In[5]:=
>>>> f = (o - 8)^4 - (e + 4)^8
>>>>
>>>> Out[5]=
>>>> -(4 + e)^8 + (-8 + o)^4
>>>>
>>>> In[6]:=
>>>> ff = Expand[f]
>>>>
>>>> Out[6]=
>>>> -61440 - 131072*e - 114688*e^2 - 57344*e^3 - 17920*e^4 - 3584*e^5 -
>>>> 448*e^6 - 32*e^7 - e^8 - 2048*o + 384*o^2 - 32*o^3 + o^4
>>>>
>>>> Is it possible to simplify ff to f again?
>>>>
>>>> Thanks!
>>>>
>>>>
>>>
>>> I don't think Mathematica can do it automatically, because the  
>>> most obvious way of carrying out this simplification relies on  
>>> transformations of the kind that  that Simplify or FullSimplify  
>>> never use (such as adding and simultaneously subtracting some  
>>> number, then rearranging the whole expression and factoring parts  
>>> of it). These kind of transformations could be implemented but  
>>> they would work in only a few cases, and would considerably  
>>> increase the time complexity of simplifying. Here is an example  
>>> of another  transformation that will work in this and some  
>>> similar cases:
>>>
>>> transf[f_, {e_, o_}] :=
>>>  With[{a = Integrate[D[f, e], e], b = Integrate[D[f, o], o]},
>>>   Simplify[a] + Simplify[b] + Simplify[(f - (a + b))]]
>>>
>>> (one can easily implment a version with more than two variables).
>>>
>>> This works in your case:
>>>
>>> ff = Expand[(o - 8)^4 - (e + 4)^8];
>>>
>>> transf[ff, {e, o}]
>>>  (o - 8)^4 - (e + 4)^8
>>>
>>> and in quite many cases like yours :
>>>
>>> gg = Expand[(a + 3)^5 + (x - 7)^6];
>>>
>>> transf[gg, {a, x}]
>>>  (x - 7)^6 + (a + 3)^5
>>>
>>> but not in all
>>>
>>>  hh = Expand[(x - 5)^4 + (y - 3)^3];
>>>
>>>  transf[hh, {x, y}]
>>>  (x - 5)^4 + y*((y - 9)*y + 27) - 27
>>>
>>> Even this, however,  is better than the answer FullSimplify gives:
>>>
>>> FullSimplify[hh]
>>>
>>> (x - 10)*x*((x - 10)*x + 50) + y*((y - 9)*y +  27) + 598
>>>
>>> Unfortunately transf also has very high complexity (it uses  
>>> Integrate)  and is unlikely to be useful in cases other than sums  
>>> of polynomials in different variables (without "cross terms") so  
>>> I doubt that it would be worth implementing some version of it in  
>>> FullSimplify.
>>>
>>>
>>> Andrzej Kozlowski
>>>
>>
>> I forgot that Integrate already maks use of Simplify, so  
>> (probably) the folowing version of transf will work just as well  
>> and faster:
>>
>> transf[f_, {e_, o_}] :=
>>  With[{a = Integrate[D[f, e], e], b = Integrate[D[f, o], o]},
>>   a + b + Simplify[(f - (a + b))]]
>>
>> Andrzej Kozlowski
>
> Sorry, that last remark just not true. Cleary Integrate uses  
> Simplify before integration but does not Simplify its output  
> (obviously to save time) so including Simplify does make a  
> difference. Note also the following works much better, but is,, of  
> course, much more tiem consuming:
>
>
> transf[f_, {e_, o_}] :=
>  With[{a = Integrate[D[f, e], e], b = Integrate[D[f, o], o]},
>   FullSimplify[Simplify[a] + Simplify[b] + Simplify[(f - (a + b))],
>    ExcludedForms -> {(x + c_)^n_, (y + d_)^m_}]]
>
>
> This will deal with many cases that would not work before:
>
> hh = Expand[(x - 5)^4 + (y - 3)^3];
>
>  transf[hh, {x, y}]
>  (x - 5)^4 + (y - 3)^3
>
>  p = Expand[(x - 2)^4 + (y + 1)^2];
>
>  transf[p, {x, y}]
>  (x - 2)^4 + (y + 1)^2
>
> but at least one power has to be higher than 3. This this will work
>
> q = Expand[(x - 2)^4 +  (y + 1)^3];
>  transf[q, {x, y}]
>  (x - 2)^4 + (y + 1)^3
>
> but this will not (all powers are <=3)
>
> q = Expand[(x - 2)^3 + (y + 1)^3];
>
> transf[q, {x, y}]
>
> x*((x - 6)*x + 12) + y*(y*(y + 3) + 3) - 7
>
> Andrzej Kozlowski
>
>
>

Sorry, another correction; it should have been (of course)


> transf[f_, {e_, o_}] :=
>  With[{a = Integrate[D[f, e], e], b = Integrate[D[f, o], o]},
>   FullSimplify[Simplify[a] + Simplify[b] + Simplify[(f - (a + b))],
>    ExcludedForms -> {(e + c_)^n_, (o + d_)^m_}]]

Andrzej Kozlowski


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