MathGroup Archive 2010

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Dot product confusion

  • To: mathgroup at smc.vnet.net
  • Subject: [mg109957] Re: Dot product confusion
  • From: dh <dh at metrohm.com>
  • Date: Wed, 26 May 2010 07:09:50 -0400 (EDT)
  • References: <htg90l$k7h$1@smc.vnet.net>

Hi Steve,
the "." product is defined as:
Sum[x1[[..,i]] x2[[i,..]],{i,1,n}]
that is the last index of x1 is contracted with the first index of x2.
 From this it is clear that commutivity only exists if x1 and x2 are 
cheers, Daniel

Am 25.05.2010 12:32, schrieb S. B. Gray:
> 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. They evaluate respectively as:
>
> {aa x1 + bb x2 + cc x3, aa y1 + bb y2 + cc y3,
> aa z1 + bb z2 + cc z3}
>
> {aa x1 + bb y1 + cc z1, aa x2 + bb y2 + cc z2,
>    aa x3 + bb y3 + cc z3}
>
> Since ptsa is itself three xyz coordinates, the expressions might be
> ambiguous, but I assumed the dot product would always commute. Should
> there be a warning?
>
> The first result is the one I want.
>
> Steve Gray
>


-- 

Daniel Huber
Metrohm Ltd.
Oberdorfstr. 68
CH-9100 Herisau
Tel. +41 71 353 8585, Fax +41 71 353 8907
E-Mail:<mailto:dh at metrohm.com>
Internet:<http://www.metrohm.com>



  • Prev by Date: Re: Scroll content inside InputField?
  • Next by Date: Re: Dot product confusion
  • Previous by thread: Re: Dot product confusion
  • Next by thread: Re: Dot product confusion