Re: Differentiating functions of vectors
- To: mathgroup at smc.vnet.net
- Subject: [mg9479] Re: [mg9424] Differentiating functions of vectors
- From: Allan Hayes <hay at haystack.demon.co.uk>
- Date: Thu, 6 Nov 1997 02:40:24 -0500
- Sender: owner-wri-mathgroup at wolfram.com
Loris Bennett <loris at ferro.physik.TU-Berlin.DE> [mg9424] Differentiating
functions of vectors
>If I have a function such as
f[ a_ , b_ ] := a . b
> how do I differentitate w.r.t a certain component without having to
> specify the components of the vector explicitly? I had hoped
> something like
fa[ a_ , b_ ] = D[ f [ a . b ] , a[[1]]]
> would work, but it doesn't.
Loris;
Two slight changes in your code seem to do the trick in this example-
but you probably have more complicated situations in mind:
In[1]:=
f[u_,v_]:= u.v;
In[2]:=
fa[ a_ , b_ ] := D[ f [ a , b ] , a[[1]]]
(*:= for =, and , for .*)
In[3]:=
a= {x,y,z};
In[4]:=
b= {p,q,r};
In[5]:=
fa[ a, b ]
Out[5]=
p
Allan Hayes
hay at haystack.demon.co.uk
http://www.haystack.demon.co.uk/training.html voice:+44 (0)116 2714198
fax: +44 (0)116 2718642
Leicester, UK