Re: Vectors
- To: mathgroup at smc.vnet.net
- Subject: [mg13953] Re: Vectors
- From: siegman at ee.stanford.edu (AES)
- Date: Fri, 11 Sep 1998 15:06:43 -0400
- Organization: Stanford University
- References: <6ssuku$m7j@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
In article <6ssuku$m7j at smc.vnet.net>, Olivier Georg
<ogeorg at imtsg12.epfl.ch> wrote:
* Hi,
*
* I want to do symbolic calculation with vectors, in particular, I want
to * do a vector product. But if I declare: *
* A = (a b c)
*
* and do
*
* A x A
*
* (which should give zero), but instead I get: *
* Cross::"nonn1": "Not all of the arguments are vectors * of
required length
You're running into the always bewildering notational inconsistencies of
Mathematica. Consider
A = ={{a,b,c}}
B = {{d},{e},{f}}
(note bracketing). Are these vectors? Well, they can be dotted, as in
A . B
and
B . A
and you'll get the right answer in each case. So, if they can be
"dotted", they ought to be "cross-able" as well, right? So try
Cross[A,B]
Cross[B,A]
Cross[A,A]
Cross[B,B]
and find that _none_ of these work. Now, page 115 of the Bible says
that Cross[a,b] can also be input as a * b, so try instead
A * B
B * A
A * A
B * B
and get three or four different answers; some of these work, some don't.
Just don't expect consistency!