? D[f,{x,n}]

*To*: mathgroup at smc.vnet.net*Subject*: [mg25495] ? D[f,{x,n}]*From*: Jack Goldberg <jackgold at math.lsa.umich.edu>*Date*: Thu, 5 Oct 2000 23:50:15 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

Hi folks, I am touching up a package I have worked on for a number of years. Rather than send it off as is, I keep tweeking it to improve its usefulness. I should know better... My latest tweek leads me to a problem with D[f,{x,n}]. Here's the story: In my package I define a constant "flag" which I set true: flag = True. Then I Unprotect D: Unprotect[D]: I want to modify f before the built-in D fires so I write a little program: D[f_,x_]/;flag := Block[ {flag=False}, D[ mySimplification[f],x] ] So, what I think happens is this. Since flag = True and since Mathematica uses my definition of D before its own, this little snippet fires. Block sets flag = False, mySimplification[f] fires, and my definition of D does not fire again (or I would be caught in a loop) so Mathematica's definition of D now works and I have accomplished my aim which is to simplify f before taking the derivative. This seems to work. However, this idea fails to work for D[f,{x,n}]! Here's my code: D[f_,{x_,n_}]/;flag := Block[ {flag=False}, D[ mySimplification[f],{x,n} ] ] (Of course, the nature of mySimplification should be irrelevant.) While trying to work out this problem, I discovered that I don't understand the difference between D[f,x] and D[f,{x,1}]. If I alter the definition of D[f,x] have I also altered the definition of D[f,{x,1}]? How about the other way around? If I alter the definiton of D[f,{x,1}] does this alter the definition of D[f,x]? It seems as though they should be linked internally but if so, I can find no reference to this linkage. This not a mute point since I found that in trying to use mySimplification in both D[f,x] and D[f,{x,n}] it is necessary to know what happens if a user chooses to write D[f,x] in the form D[f,{x,n}]. Help is needed. Thanks.

**Follow-Ups**:**Re: ? D[f,{x,n}]***From:*Carl Woll <carlw@u.washington.edu>

**RE: making a function linear**

**Exporting multiple page graphics**

**Re: Syntax**

**Re: ? D[f,{x,n}]**