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!