Re: Re:Re: FullSimplify with Assumptions

*To*: mathgroup at smc.vnet.net*Subject*: [mg53449] Re: [mg53435] Re:Re: FullSimplify with Assumptions*From*: Adam Strzebonski <adams at wolfram.com>*Date*: Wed, 12 Jan 2005 03:41:21 -0500 (EST)*References*: <200501110631.BAA00751@smc.vnet.net> <AC2C4308-63DC-11D9-94CC-000A95B4967A@mimuw.edu.pl>*Reply-to*: adams at wolfram.com*Sender*: owner-wri-mathgroup at wolfram.com

Alternatively, one can try FullSimplify with all possible orderings of chosen variables. Of course, this multiplies the computation time by Factorial[Length[variables]]... Here is a simple implementation of a variable order independent FullSimplify. In[1]:= VOISimplify[vars_, expr_, assum_:True] := Module[{perm, ee, best}, perm=Permutations[vars]; ee=(FullSimplify@@({expr, assum}/.Thread[vars->#]))&/@perm; best=Sort[Transpose[{LeafCount/@ee, ee, perm}]][[1]]; best[[2]]/.Thread[best[[3]]->vars]] In[2]:= VOISimplify[{x, y}, (L - L*y^2)/x^2, {-1 + x^2 + y^2 == 0}] Out[2]= L In[3]:= VOISimplify[{x, y}, (L - L*x^2)/y^2, {-1 + x^2 + y^2 == 0}] Out[3]= L Best Regards, Adam Strzebonski Wolfram Research Andrzej Kozlowski wrote: > > On 11 Jan 2005, at 07:31, Goyder Dr HGD wrote: > >> Thank you for your comments on my questions. However, this raises more >> questions: >> >> 1. What is the mission of FullSimplify? I thought the mission was to >> minimise the LeafCount of the expression. Are you are saying that this >> is not possible if assumptions are included? If this is not possible >> then what can FullSimplify hope to do? > > > FullSimplify tries to simplify an expression according to a user > specified complexity function and using user specified transformation > functions. Of course it uses certain defaults, which are selected > because they have been judged most suitable for most users. You can > inspect all the options of FullSimplify: > > > Options[FullSimplify] > > > {Assumptions :> $Assumptions, ComplexityFunction -> > Automatic, ExcludedForms -> {}, > TimeConstraint -> Infinity, TransformationFunctions -> > Automatic, Trig -> True} > > As you can see the the default ComplexityFunction and > TransformationFunctions are not specified (their value is Automatic) but > Adam Strzebonski once posted the code of the default ComplexityFunction > to this list. Any way, it is not actually LeafCount but is generally > close to LeafCount. But of course the default values are not going to be > suitable for most users. If they are not suited to your needs you have > to define your own ComplexityFunction and your own > TransformationFunctions. Moreover, anything that is included in the > defaults will affect the vast majority of users who normally use only > default values. It would obviously not be a good idea to choose defaults > that might be ideal for one or two users and would cause other users > programs to run much slower to no useful purpose. > > Andrzej Kozlowski > > >> >> 2. If I change the input of FullSimplify from an expression to an >> assertion to be tested, see below, then it seems to be able to use the >> assumptions. Is this always going to be true? >> >> In[3]:= >> InputForm[FullSimplify[(L - L*y^2)/x^2, {-1 + x^2 + y^2 == 0}]] >> >> Out[3]//InputForm= >> (L - L*y^2)/x^2 >> >> In[4]:= >> InputForm[FullSimplify[(L - L*y^2)/x^2 == L, {-1 + x^2 + y^2 == 0}]] >> >> Out[4]//InputForm= >> True >> > > In this kind of cases (purely algebraic ones) it will always be true > since in this case the answer is independent of the Groebner basis > used. What you are doing in essentially: > > > > Last[PolynomialReduce[(L-L*y^2)/x^2-L,{-1+x^2+y^2},{x,y}]] > > > 0 > > > Last[PolynomialReduce[(L-L*y^2)/x^2-L,{-1+x^2+y^2},{y,x}]] > > > 0 > > and the answer independent of the Groebner basis used even though, as I > pointed out earlier: > > > Last[PolynomialReduce[(L - L*y^2)/x^2, {-1 + x^2 + y^2}, > {x, y}]] > > L/x^2 - (L*y^2)/x^2 > > > Last[PolynomialReduce[(L - L*y^2)/x^2, {-1 + x^2 + y^2}, > {y, x}]] > > L > >> 3. Why is it so impossible to check all lexical orderings? How would >> the time increase with the number of variables and the number of >> assumptions? Should this be an option, with warnings and time >> constraints? > > > It is possible but you have to do it yourself. You can append suitable > transformation functions to the default transformation functions. > > >> >> 4. The actual source of my problem was a huge trig function. I had >> changed the trig functions into polynomials and the assumption above >> was the remains of Sin[t]^2 + Cos[t]^2 == 1. FullSimplify did a lot of >> good work before falling at the last hurdle. How else can one proceed? > > > The defaults are meant to be helpful to most people. They are not meant > to solve all problems and clearly it would not be possible to to do that > no matter what defaults where chosen. When they do not work in your > particular situation you have to write your own ComplexityFunction and > choose your own TransformationFunctions. > > >> >> Thanks >> >> Hugh Goyder >> -----Original Message----- >> From: Adam Strzebonski [mailto:adams at wolfram.com] To: mathgroup at smc.vnet.net >> Subject: [mg53449] [mg53435] Re: [mg53339] FullSimplify with Assumptions >> >> >> Andrzej Kozlowski wrote: >> >>> *This message was transferred with a trial version of CommuniGate(tm) >>> Pro* >>> On 7 Jan 2005, at 12:00, Goyder Dr HGD wrote: >>> >>>> In the examples below I would expect FullSimplify to give L. >>>> However, I get results that depend on the symbols I use. >>>> It would appear that symbols x and y are treated differently. >>>> >>>> How can I force FullSimplify to use the LeafCount as the >>>> ComplexityFunction? >>>> >>>> >>>> In[14]:= r1 = FullSimplify[(L - L*y^2)/x^2, {-1 + x^2 + y^2 == 0}] >>>> >>>> Out[14]= (L - L*y^2)/x^2 >>>> >>>> In[15]:= r2 = FullSimplify[(L - L*x^2)/y^2, {-1 + x^2 + y^2 == 0}] >>>> >>>> Out[15]= L >>>> >>>> In[16]:= LeafCount[r1] >>>> >>>> Out[16]= 12 >>>> >>>> In[17]:= LeafCount[r2] >>>> >>>> Out[17]= 1 >>>> >>>> In[18]:= $Version >>>> >>>> Out[18]= "5.1 for Microsoft Windows (October 25, 2004)" >>>> >>>> Thanks for any comment >>>> >>>> Hugh Goyder >>>> >>> I would speculate that the reason has something to do with FullSimplify >>> using PolynomialReduce or related functions, whose outcome depends on >>> the ordering of the variables. Compare for example: >>> >>> >>> Last[PolynomialReduce[(L - L*y^2)/x^2, {-1 + x^2 + y^2}, >>> {x, y}]] >>> >>> >>> L/x^2 - (L*y^2)/x^2 >>> >>> with >>> >>> >>> Last[PolynomialReduce[(L - L*y^2)/x^2, {-1 + x^2 + y^2}, >>> {y, x}]] >>> >>> L >>> >>> Note also that if you do not include explicitly the variables you >>> will get: >>> >>> >>> Last[PolynomialReduce[(L - L*y^2)/x^2, {-1 + x^2 + y^2}]] >>> >>> >>> L/x^2 - (L*y^2)/x^2 >>> >>> >>> I suspect this isn't a bug in FullSimplify. To remedy it FullSimplify >>> would need to use all possible orderings of variables when applying >>> algebraic functions whose output depends on variable order, and that is >>> obviously just not a reasonable option in terms of performance. Also, >>> Simplify and FullSimplify are not really optimized for conditions of the >>> form something == something else; I find it a little surprising that it >>> works as well as it does. One should really deal with such problems by >>> using polynomial algebra, that is PolynomialReduce etc, with specified >>> variable ordering. >>> >>> >>> Andrzej Kozlowski >>> Chiba, Japan >>> http://www.akikoz.net/~andrzej/ >>> http://www.mimuw.edu.pl/~akoz/ >>> >> >> This is exactly the case. >> >> When FullSimplify is given equational assumptions it >> computes a single Groebner basis of the assumptions >> (with the DegreeReverseLexicographic monomial order >> and with the variables ordered by Sort). >> PolynomialReduce with respect to this Groebner basis >> is then applied as one of the transformation functions >> used to simplify subexpressions. >> >> Best Regards, >> >> Adam Strzebonski >> Wolfram Research >> >> >> >> >> -- >> This message has been scanned for viruses and >> dangerous content by the Cranfield MailScanner, and is >> believed to be clean. >> >

**References**:**Re:Re: FullSimplify with Assumptions***From:*"Goyder Dr HGD" <h.g.d.goyder@cranfield.ac.uk>

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