ComplexExpand, Piecewise, and simplification

*To*: mathgroup at smc.vnet.net*Subject*: [mg66659] ComplexExpand, Piecewise, and simplification*From*: Andrew Moylan <andrew.moylan at anu.edu.au>*Date*: Fri, 26 May 2006 04:17:19 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

Thanks for your help regarding "Simplification and Arg[]", Andrzej and others. I have written a function that determines which independent variables in an expression are not known to be real (according to $Assumptions), and thus must be passed as the second argument to ComplexExpand. This makes ComplexExpand safe to call on any expression. I have since noticed that FullSimplify is also comparitively poor at handling Piecewise functions. There are two aspects to this that I have found: (1) In an expression like a+Piecewise[{{b,condition1},{c,condition2}}], it is frequently useful to try calling PiecewiseExpand; it will bring the "a+" inside the Piecewise function and allow for simplification/cancellation of it with the pieces. This is a very cheap operation, but FullSimplify does not appear to take advantage of it. (2) In an expression like Piecewise[{{b,condition1},{c,condition2}},defaultvalue], it is frequently useful to append condition1 to $Assumptions and then call FullSimplify on piece b, followed by instead appending condition2 to $Assumptions and calling FullSimplify on piece c, then finally appending Not[Or[condition1,condition2]] to $Assumptions and then calling FullSimplify on the defaultvalue. As Andrezj suggested, I have written extended versions of Simplify and FullSimplify. They expose an indentical interface to the built-in functions, except they expose additional options specifying whether to try various ComplexExpand transformation functions (with various values for the TargetFunctions option and automatically-determined list of complex variables); and whether to use the simple Piecewise simplification methods described above. -Andrew Andrzej Kozlowski wrote: > On 22 May 2006, at 11:30, Andrew Moylan wrote: > > >>Should Mathematica be able to simplify the following expression? >>(It is >>easily seen to be zero under the given condition, x > 0.) >> >>FullSimplify[ >> Arg[1 + I * x] + Arg[1 - I * x], >> {x > 0} >>] >> >>In particular, I would have expected the following to yield ArcTan[b / >>a], from which the above expression is easily reduced to zero: >> >>FullSimplify[ >> Arg[a + I b], >> {a > 0, b > 0} >>] >> >>Any ideas? >> >>Cheers, >> >>Andrew >> >>P.S. Apologies if I have sent this twice; my original message seems >>not >>to have worked. >> > > > > You do not even need the condition x>0: it is enough that x is real. > > ComplexExpand[Arg[1 + I * x] + Arg[1 - I * x],TargetFunctions->{Re,Im}] > > 0 > > Simplify and FullSimplify by default do not make use of ComplexExpand. > Of course, if you wish you can make Simplify use ComplexExpand: > > Simplify[Arg[1 + I*x] + Arg[1 - I*x], > TransformationFunctions -> > {ComplexExpand[#1, TargetFunctions -> {Re, Im}] & , > Automatic}] > > 0 > > Note that doing this automatically involves the assumption that x is > real, so it would not be a good idea to permanently append > ComplexExpand to the TransformationFunctions, except when the > assumptions imply that the variables involved are real. However, it > is not difficult to write a version of Simplify or FullSimplify which > will make use ComplexExpand in a way that is compatible with the > assumptions about the variables (real or complex). That it is not > done by default is probably due to the facts that, on the one hand, > ComplexExpand is a high complexity function, and on the other, it > usually leads to more rather than less complex expressions (after all > it "Expands"). > > Note also that: > > > ComplexExpand[Arg[a + I*b], TargetFunctions -> {Re, Im}] > > > ArcTan[a, b] > > so the assumption about a and b being positive are again not needed. > > > Andrzej Kozlowski > Tokyo, Japan >

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