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Re: Dot product confusion

  • To: mathgroup at smc.vnet.net
  • Subject: [mg109952] Re: Dot product confusion
  • From: Mark Adler <madler at alumni.caltech.edu>
  • Date: Wed, 26 May 2010 07:08:56 -0400 (EDT)
  • References: <htg90l$k7h$1@smc.vnet.net>

On 2010-05-25 03:32:53 -0700, S. B. Gray said:
> ptsa = {{x1, y1, z1}, {x2, y2, z2}, {x3, y3, z3}};

The dot product of two vectors commutes, but your ptsa is a matrix.  In
general matrix multiplication does not commute.

> {aa, bb, cc}.ptsa  (* expression 1 *)
> ptsa.{aa, bb, cc}  (* expression 2 *)

Mathematica treats ptsa as a column vector (1x3) in expression 1, and
as a row vector (3x1) in expression 2.

Mark


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