Re: Reduce an expression into a given form.

• To: mathgroup at smc.vnet.net
• Subject: [mg130914] Re: Reduce an expression into a given form.
• From: Bill Rowe <readnews at sbcglobal.net>
• Date: Sat, 25 May 2013 05:38:39 -0400 (EDT)
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```On 5/24/13 at 6:26 AM, francesco.cordoni88 at gmail.com (Francesco
Cordoni) wrote:

>I have an expression of the form N[a,b,c]/D[a,b,c] with a, b and c
>some parameters on which N and D depend. I have to transform it into
>something like A[a,b]/c with A that does not depend on c. Is there
>any function in Mathematica that can do that? Thanks a lot for any
>help.

No in that you cannot use N and D as you attempt to do above.
These have built-in meaning and if you paste N[a,b,c] into a
notebook with default settings you will see the last comma and c
are colored red indicating a syntax error. And D[a, b, c] gets
evaluated to zero.

It is impossible to say anything meaningful beyond the remarks
above until you define your functions.

Note, I can always do something like:

In[6]:= n[a, b, c]/d[a, b, c] /. {n[x_, y_, _] -> k[x, y],
d[_, _, x_] -> x}

Out[6]= k[a, b]/c

But since this type of replacement is akin to replacing
characters in a string and is not doing any mathematics, it
cannot be guaranteed to have any mathematical validity. And in
fact since the functions n,d and k are undefined, the result is meaningless.

```

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