Re: bug? f'[x]'
- To: mathgroup at smc.vnet.net
- Subject: [mg90911] Re: [mg90894] bug? f'[x]'
- From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
- Date: Tue, 29 Jul 2008 01:39:07 -0400 (EDT)
- References: <200807281153.HAA20595@smc.vnet.net>
On 28 Jul 2008, at 13:53, Benjamin.R.Lewis at gmail.com wrote: > G'day, > > In[]:= f'[x] =!= f[x]' && f''[x] === f'[x]' > Out[]= True > > Can anyone explain this? > > -Ben > Maybe not exactly explain... but this is a consequence of the following somewhat strange phenomenon: FullForm[Derivative[n][Derivative[m][f][x]]] FullForm[Derivative[m + n][f][x]] but not for m=0: FullForm[Derivative[n][Derivative[0][f][x]]] FullForm= FullForm[Derivative[n][f[x]]] Note that the first output is actually the (m+n)-th derivative of f at x, but the second output means (if anything) something completely different (n-th derivative of the functon f[x]). Although the first result may be part of some design, it is mathematically just as meaningless as the second. So I would guess the "explanation" of the phenomenon you have observed is of the "garbage in -> garbage out" kind. Andrzej Kozlowski
- References:
- bug? f'[x]'
- From: Benjamin.R.Lewis@gmail.com
- bug? f'[x]'