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Re: Why D[x', x] isn't 0 ?

• To: mathgroup at smc.vnet.net
• Subject: [mg66123] Re: Why D[x', x] isn't 0 ?
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Sun, 30 Apr 2006 04:21:29 -0400 (EDT)
• References: <e2v6im$ncb$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

Hi,

and why not write

D[x'[t], x[t]]

and use the Mathematica syntax ?
Because it is quite common that computer
programs respond nonsense when the user
enters nonsense.

Regards
   Jens

Vladimir wrote:
> Hello!
> 
> D[x'', x']  =>  0  (* okay *)
> 
> D[x', x]  =>  x''  (* unexpected despite being similar to above *)
> 
> D[f[x'], x]  =>  x'' f'[x']  (* unexpected as well *)
> 
> Why the last 2 lines don't produce zeroes and how to make them
> do so (for any function f) because it's quite common in mechanics
> where velocity is treated as independent of position, etc.
> 
> --
> Vladimir
>