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