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Re: On constants and D's behaviour


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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