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Re: Dot product confusion

  • To: mathgroup at smc.vnet.net
  • Subject: [mg109964] Re: Dot product confusion
  • From: Bill Rowe <readnews at sbcglobal.net>
  • Date: Wed, 26 May 2010 07:11:07 -0400 (EDT)

On 5/25/10 at 6:32 AM, stevebg at ROADRUNNER.COM (S. B. Gray) wrote:

>Given

>ptsa = {{x1, y1, z1}, {x2, y2, z2}, {x3, y3, z3}};

>I  thought the following expressions would be identical:

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

>but they are not.

>Since ptsa is itself three xyz coordinates, the expressions might be
>ambiguous, but I assumed the dot product would always commute.

The dot product of two *vectors* commutes. But you are
multiplying a matrix by a vector which does not commute in general

>Should there be a warning?

No. Until Mathematica is run on a machine with artificial
intelligence and has the capability of reading minds, there
isn't any way for Mathematica to determine the user expected a
result different from the correct mathematical result.



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