Re: Re: Re: Re: Re: Forcing a Derivative

*To*: mathgroup at smc.vnet.net*Subject*: [mg50850] Re: [mg50835] Re: [mg50800] Re: [mg50793] Re: [mg50778] Re: Forcing a Derivative*From*: Andrzej Kozlowski <andrzej at akikoz.net>*Date*: Fri, 24 Sep 2004 04:41:24 -0400 (EDT)*References*: <cijej8$hlp$1@smc.vnet.net> <200409200139.VAA27554@smc.vnet.net> <200409210749.DAA27779@smc.vnet.net> <200409220410.AAA18643@smc.vnet.net> <200409230927.FAA08049@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

My point was slightly different. What I meant was that in Mathematica almost everything is, in a sense, a function and almost everything can be an argument of a function, in that you can write f[a] where f and a can be almost any symbol. And the whole issue is whether one wants this: Through[(f+g)[a]] f[a]+g[a] to happen automatically even when Through is not present (similarly with Times etc). Once you modify Plus, Times and Power in the obvious way so that this happens you will get the behaviour for derivatives you seem to want automatically. However, as I wrote earlier, I am not entirely convinced that it is desirable that this sort of thing should happen automatically. I can't give at this moment a sensible, concrete example of any inconvenience this behaviour would cause but it would further impose certain interpretation on what are after all only abstract symbols and might make it harder to use them in a different way (in other words, one may be able to think of contexts when one does not want (f+g)[x] to be automatically converted to f[x] +g[x]). I think of Mathematica essentially as an "algebraic programming language", rather than a program designed with specific mathematical applications, e.g complex analysis, number theory etc. in mind. In a programming language flexibility is valuable, if some particular behaviour is too much tied to a particular application or context it will make it harder to apply it to other applications or contexts. Of course there is always the need to find a balance: note for example that the Mathematica evaluator will expand this automatically (a*b)^3 a^3*b^3 but not this (a + b)^3 (a + b)^3 There are lot's of choices of this kind that have to be made and even in the simple case of the first example above you will find that there are from time to time users who find it inconvenient. All these are matters of design and in most cases Ithink whatever choices are made someone will think they are the wrong ones. Andrzej Kozlowski On 23 Sep 2004, at 18:27, Murray Eisenberg wrote: > *This message was transferred with a trial version of CommuniGate(tm) > Pro* > I agree that Mathematica's notion of function MAY be broader than in > Mathematics in some respects (but in others is MUCH narrower -- that's > quite another topic). But I don't quite see how that necessarily > precludes a reasonable implementation of such an algebra of > numerically-valued functions of numerical arguments. > > After all, I would not try to evaluate > > Plot'[x] > > either! But that doesn't prohibit me from evaluating, say, Sin'[x]. > > Likewise, just because I would not dream of asking for the > syntactically > correct expression > > Plot + NumberForm > > to have meaning does not preclude a meaning for, say, Sin + Exp. > > > > Andrzej Kozlowski wrote: > >> But one does not need to introduce x in f: >> >> f[x_]:=x^3 >> >> >> Derivative[2][f] >> >> 6 #1& >> >> No x was needed. >> >> As has been pointed out already a number of times, what is not >> implemented by default is the algebra of complex functions, that is, >> if f and g are functions then 2f + 3 g or 5 f*g are not considered by >> Mathematica to be functions. One reason for that maybe that in >> Mathematica the notion of a "function" is broader than in Mathematics. >> In any case the algebras of functions and operators are easy to >> implement oneself and this has already been done more than once on >> this >> list.... > > > -- > Murray Eisenberg murray at math.umass.edu > Mathematics & Statistics Dept. > Lederle Graduate Research Tower phone 413 549-1020 (H) > University of Massachusetts 413 545-2859 (W) > 710 North Pleasant Street fax 413 545-1801 > Amherst, MA 01003-9305 >

**References**:**Re: Forcing a Derivative***From:*Karl_boehme_9@msn.com (Klaus G)

**Re: Re: Forcing a Derivative***From:*Murray Eisenberg <murray@math.umass.edu>

**Re: Re: Re: Forcing a Derivative***From:*Andrzej Kozlowski <andrzej@akikoz.net>

**Re: Re: Re: Re: Forcing a Derivative***From:*Murray Eisenberg <murray@math.umass.edu>

**Re: Timing of statements inside a cell**

**help on Rewrite rules**

**Re: Re: Re: Re: Forcing a Derivative**

**Re: Forcing a Derivative**