Re: On constants and D's behaviour

• To: mathgroup at smc.vnet.net
• Subject: [mg52470] Re: On constants and D's behaviour
• From: David Bailey <dave at Remove_Thisdbailey.co.uk>
• Date: Sun, 28 Nov 2004 01:07:00 -0500 (EST)
• References: <co97pg\$gub\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Nicolas Girard wrote:
> Dear people,
> just a thought, from a frustrated mind:
>
> given that one can explicitely tell a symbol is a constant by modifying
> its attributes, then why the hell doesn't the partial derivative D take
> advantage of it, and treat nonconstant symbol as.... nonconstants, by
> default ??
>
> Cheers,
> Nicolas
>

I suspect part of the problem with D and Dt is that a set of conventions
that might suit one problem domain would not seem right in another
context. In any case, I am sure the rules for these operations are cast
in stone because too much stuff would break if WRI changed them! I must
say though (relating to your previous question) I have never understood
why D evaluates if either of its arguments is a pattern!

It is, however, very easy to supply a recursive definition for dD that
uses whatever rules you see fit. One option is to define dD using a set
of transformation rules and then apply them with //. if and when you
want your derivatives evaluated. That way you can leave expressions like
d(sin(x))/dx unevaluated in your expressions, which can be useful in
some contexts.

David Bailey

```

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