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 >