Re: solving vector equations in mathematica
- To: mathgroup at smc.vnet.net
- Subject: [mg34366] Re: solving vector equations in mathematica
- From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
- Date: Fri, 17 May 2002 06:30:46 -0400 (EDT)
- Organization: Universitaet Leipzig
- References: <abvu3q$m4b$1@smc.vnet.net>
- Reply-to: kuska at informatik.uni-leipzig.de
- Sender: owner-wri-mathgroup at wolfram.com
Hi,
x[[i]] is not a indexd symbol it is the *part* of an existing
expression.
you mean
Solve[Sum[Subscript[x, i], {i, 1, 5}] == 0, Subscript[x, 2]]
Regards
Jens
Hal Daume III wrote:
>
> Hi All,
>
> I'm somewhat new to mathematica, at least in the world of vectors, and
> I'm having trouble getting it to solve an equation for me.
>
> I basically want to solve:
>
> \sum_{i=1}^n x_i = 0
>
> for x_k
>
> I tried:
>
> Solve[Sum[x[[i]],{i,1,5}] == 0, x[[2]]]
>
> as an example, but mathematica complains:
>
> In[118]:= Solve[Sum[w[[i]],{i=1,5}] == 0, w[[1]]]
>
> Sum::write: Tag Set in i = 1 is Protected.
>
> Part::partd: Part specification w[[1]] is longer than depth of object.
>
> Part::partd: Part specification w[[1]] is longer than depth of object.
>
> Part::partd: Part specification w[[1]] is longer than depth of object.
>
> General::stop: Further output of Part::partd
> will be suppressed during this calculation.
>
> Out[118]= {{w[[1]] -> 0}}
>
> I'm not exactly sure what this means.
>
> I could write it out:
>
> In[119]:= Solve[w1+w2+w3+w4+w5==0, w1]
>
> Out[119]= {{w1 -> -w2 - w3 - w4 - w5}}
>
> But this isn't very helpful in my situation (my equations are huge*), and
> doesn't reflect the vector quality of the situation.
>
> If such a thing is possible, I'd appreciate some pointers (I read the
> mathematica docs on the wolfram site, but couldn't find anything talking
> about this).
>
> Thanks!
>
> - Hal
>
> * the equation i'm working with is something like:
>
> sum_{C,x,y,y'} (wx . wy) (wx . wy') (|wx * wy'| (wx . wy) - |wx * wy| (wx
> . wy')) / (|wx|^4 |wy|^2 |wy'|^2) == 0
>
> where
>
> . is dot product
> * is cross product
> |x| is magnitude of x
> and
> for vectors w=<w1,w2,...,wn>, x=<x1,x2,...,xn>,
> wx = <w1*x1,w2*x2,...,wn*xn>
>
> Thanks again.
>
> --
> Hal Daume III
>
> "Computer science is no more about computers | hdaume at isi.edu
> than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume