Re: Re: Simplifying and Rearranging Expressions

*To*: mathgroup at smc.vnet.net*Subject*: [mg95981] Re: [mg95956] Re: Simplifying and Rearranging Expressions*From*: Andrzej Kozlowski <akoz at mimuw.edu.pl>*Date*: Sat, 31 Jan 2009 01:15:11 -0500 (EST)*References*: <gls1u8$hjl$1@smc.vnet.net> <200901301047.FAA06653@smc.vnet.net> <78E34FCC-E733-4CAE-AA27-CE95DB401072@mimuw.edu.pl>

Correction, not "Computation Center" but "Calculation Center" which has since been renamed CalcCenter and it seems it is still being sold: http://www.wolfram.com/products/calccenter/ Andrzej Kozlowski On 30 Jan 2009, at 16:14, Andrzej Kozlowski wrote: > To my amazement I have found something that I agree here. I do agree > that it is largely a pointless waste of time to use computer > algebra, which relies on pretty complex algorithms (like Groebner > basis) to make this **look** the way you want them to look, for > reasons that have no particular relation to these algorithms. Its a > waste of computing resources, the effort of mathematicians and > programmers and most of all the user, who could much more easily > achieve this effect in other ways, one of which is TeX (even better > was, in my opinion, was David Bailey's clever idea to use color to > manipulate Mathematica expressions almost as one does by hand - > unfortunately this appears to have been abandoned due to lack of > interest). > I don't think however there is any chance whatever of WRI > incorporating TeX into Mathematica, for two reasons. One is that it > would be going against their principal idea of having all > Mathematica expressions fully controllable by means of the > Mathematica programming language. Clearly this would not be true of > TeX strings, if they were meant to be interpreted for display. > Secondly, because other CAS systems have essentially tried to do > this sort of thing with very little to show for it in terms of > market success. You seem to be completely unaware of how tiny the > TeX users community is compared with the community of users of > programs like Mathematica. > This reminds me also that a lot of suggestions which you have made > about the way Mathematica ought to be (simple, cheap, computation > engine, no fancy staff) has already been tried by WRI and clearly > failed. It was called something like The Computation Center, and > limited version of Mathematica, that Wolfram once sold for a > fraction of the price of the full thing. The only problem was that > hardly anyone bought it (I suspect you did not either). > Not surprisingly WRI is likely to be pretty skeptical of bright > ideas that remind them of things that they or others have already > tired and have been shown not to work. > > Andrzej Kozlowski > > On 30 Jan 2009, at 11:47, AES wrote: > >> In article <gls1u8$hjl$1 at smc.vnet.net>, >> "David Park" <djmpark at comcast.net> wrote: >> >>> I want to start a thread on this because I believe many MathGroup >>> people >>> will have some useful things to say. >> >> I'll bite, because I've done a bit of thinking on this. >> >> >>> A common task for Mathematica users is to obtain an expression >>> that is in a >>> particular form. For students and teachers this may often be a >>> textbook >>> form, or there may be other reasons that a particular form is >>> desired. >> >> Just to add a bit of specificity to this, let's consider expressions >> that arise in optics and e-m theory, which generally involve a set of >> physical quantities (velocity of light, propagation constant, >> permeabilities, index of refraction, frequency, wavelength, >> characteristics impedance, critical angle of refraction, and multiple >> others) that are conventionally written as >> >> c, k, (or beta), mu, epsilon, n, f (or omega), lambda, >> eta or z_0, thetaCrit, and so on >> >> **each of which is directly linked or coupled to (that is, can be >> calculated from) several others in the same set**. >> >> [You have to put up with a side story at this point. The >> distinguished >> physicist W. K. H. Panofsky, who just recently died, early in his >> career >> co-authored with Melba Phillips a small but excellent text on >> classical >> e-m, colloquially known as "Panofsky and Phillips", from which I and >> many others studied. This was long enough ago that cgs and mks units >> were still fighting it out in the physics community.] >> >> [P and P beautifully sidestepped this issue by, as they noted in the >> Preface to their book, writing every equation in their book using an >> appropriate (but generally different) subset of the above symbols, >> such >> that every equation was valid in mks units as it stood, **and could >> be >> instantly converted to be exactly valid in cgs units as well, >> simply by >> replacing any factor of epsilon that appeared in any of these >> equations >> by 1/ 4 pi **.] >> >> >>> It might be thought that this should be an easy task but quite >>> often it can >>> be a very difficult task, even involving mathematical derivation >>> and many of >>> the capabilities of Mathematica. Not obtaining a specific form may >>> be a >>> matter of not knowing how to solve the problem in the first place. >> >> It may not be just a difficult task; in fact, **it may be an >> impossible >> task** -- not to mention **an unnecessary and undesirable task**. >> >> 1) As already noted above, you may want to write expressions that >> contain some subset of a linked set of variables in different ways at >> different points in an exposition, because these different ways are >> conventional in the field, and/or make the physical meaning clearer. >> >> For example, you may want to write the space-time variation of a >> phasor >> wave amplitude as Exp[ I k z - I omega t] because that's neat, >> simple, >> and conventional. >> >> But then, in discussing a waveguide mode where a factor k d (d = >> waveguide width) appears, you may want to write that factor instead >> in >> the form 2 pi d/lambda to emphasize that it's the width in >> wavelengths >> that's important. >> >> But if at some point in your notebook you're going to insert any of >> the >> dependences within this set -- e.g., k := 2 pi / lambda -- then >> you're >> stuck with this from then on. >> >> 2) A second point: My experience has been that useful identities >> that >> often arise in analyses -- for example, with suitable >> qualifications the >> infinite integral of Exp[- a x^2 + b x] == Exp[b^2/4a] -- sometime >> just >> won't fall out (i.e., won't be explicitly evaluated by Mathematica >> if a >> and b are actually more complicated expressions. >> >> 3) More generally, one very often wants to do the eventual numerical >> calculations using only one or another form of dimensionless or >> normalized variables, because that's numerically efficient as well as >> physically and practically useful in expressing the results. >> >> And, if at some point in an exposition you're going to convert your >> analytical and expositional formulas into dimensionless formulas for >> numerical calculation purposes -- **at that point you really don't >> care >> how Mathematica arranges the resulting expression**. >> >> >>> Nevertheless, even simple rearrangement can be difficult. I >>> sometimes think >>> of it as doing surgery on expressions. I believe it is generally >>> desirable >>> to use Mathematica to rearrange an expression and not retype the >>> expression. >>> Retyping is too error prone. >> >> Last sentence is true; immediately preceding sentence may be true as >> phrased -- but is a mistaken belief. Comments on this below. >> >> >>> Simplify and FullSimplify are amazingly useful but it is difficult >>> to >>> control them and obtain a precise result. One will often have to do >>> additional piecemeal operations. One downside of Simplify and >>> FullSimplify >>> is that they can return different forms with different Mathematica >>> versions. >>> Then any additional operations in an old notebook may no longer >>> work. It >>> would be nice if there was a method of using these commands that >>> would be >>> more version independent. >> >>> Various routines such as Together, Apart, Factor, TrigReduce, >>> TrigFactor, >>> TrigExpand, TrigToExp, GroebnerBasis etc., can be useful in >>> getting a >>> specific form. MapAt is very useful for doing surgery on specific >>> parts of >>> an expression. Mathematica often gets two factors that have extra >>> minus >>> signs. You can correct that by mapping Minus onto the two factors. >>> For >>> integrals in the wrong form you could cheat by trying to find the >>> constant >>> by which they differ by subtracting and simplifying, and then use >>> that in >>> the derivation. >> >> Let's say it like it is: It's not just "difficult" for ordinary >> users >> to use and control many of these advanced tools: It's basically >> **impossible** for the average user to learn what some of these tools >> do, because they're so complex and the results can depend so >> critically >> on what you put into them; all you end up doing is thrashing around >> endlessly, trying to get them to produce the results you want. >> >> The more powerful they get, the less they're worth trying to learn. >> >> >>> It is very useful to get Mathematica generated expressions into >>> the form >>> that one wants. I believe that this is probably a sticking point >>> with many >>> users. In general it is not a trivial topic. Others may have some >>> good >>> general ideas that I don't know about. >> >> My bottom lines are instead: >> >> 1) Accept that "Retyping is...error prone" -- and more generally >> that >> "To err is human..." -- and to the extent that you have to do any >> form >> of retyping, do a _lot_ of checking, rechecking, testing with simple >> cases, and looking to see that results are physically meaningful. >> >> 2) Nonetheless, in general, "It is GENERALLY NOT very useful to get >> Mathematica generated expressions into the form that one wants" -- at >> least, not very often, and not if it involves any significant >> amount of >> effort. It's wasted energy, and can add its own errors, or divert >> one >> from seeing one's own errors. >> >> 3) Instead, if what you're doing is a complex analysis and/or >> exposition, tackle the analysis portion initially with paper, pencil, >> and a good soft eraser, the way God intended analysis to be done. >> >> 4) When and if certain calculations (series expansions, etc.) get >> messy, run separate Mathematica symbolic calculations in auxiliary >> notebooks to carry them out. >> >> 5) When it comes time for exposition, do the exposition using a tool >> that's designed for exposition (e.g., TeX), while doing the >> numerical >> calculations and graphing using a tool that's good at those things -- >> and while doing this repeat item 1) multiple times. >> >> >>> Someday someone may even write a good tutorial on it. >> >> How about instead someone **imbedding real TeX in Mathematica**, as >> part >> of Mathematica's basic capabilities? >> >> That is: >> >> * TeX is (I believe) totally open source, free, highly stable, and >> widely known and studied -- and it's full source code is very >> compact. >> >> * So how about building the TeX source code into Mathematica's >> already >> immense repertoire of rules and stuff, and allowing one to include at >> any point in a "text portion" (I.e., a "non-evaluation portion") of a >> Mathematica notebook cell the syntax >> >> TeX[ ---any valid TeX syntax---] >> >> such as TeX[$\alpha = \beta / \gamma^2$] to get that bit of inline >> math >> into a Text or Header cell, or TeX[$$\alpha = \left( \beta /over >> \gamma^2 \right)$$] to insert a display equation, >> >> and just having Mathematica display the typeset box produced by TeX >> using that syntax into the Mathematica notebook at that point, with >> the >> stuff inside the [ ] brackets having no other evaluational function >> or >> effect in Mathematica itself, except to be displayed? >> >> Is there some reason this would be conceptually impossible? Would >> it be >> that difficult to accomplish? Could it at least be implemented with >> some >> reasonable subset of TeX syntax and capabilities? >> >> If your goal is to have Mathematica notebooks serve simultaneously as >> "exposition documents" and "calculation performing documents", might >> this be a lot easier than endless fighting with option-laden and >> temporally unstable Mathematica expressions like "Together, Apart, >> Factor, TrigReduce, TrigFactor, TrigExpand, TrigToExp, GroebnerBasis" >> and all their even more arcane extensions? >> > > >

**References**:**Re: Simplifying and Rearranging Expressions***From:*AES <siegman@stanford.edu>

**Re: Re: Simplifying and Rearranging Expressions**

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