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Re: Symbolic vector handling

  • To: mathgroup at smc.vnet.net
  • Subject: [mg83408] Re: [mg83353] Symbolic vector handling
  • From: danl at wolfram.com
  • Date: Mon, 19 Nov 2007 06:23:51 -0500 (EST)
  • References: <200711180946.EAA01450@smc.vnet.net>

> Hello group,
>
> this is most probably a very simple question but still ...
>
> Consider a set of vectors vA, vB, vC, ... and scalars a, b, c, ...
>
> What I want to do is a symbolic handling of expressions like vA.(b vB +
> vC)
> which should be expanded to
>
> (1)    expr = vA.(b vB + vC) -> b vA.vB + vA.vC
>
> i.e. I want Mathematica to apply the distributive law and the
> extraction of scalars.
> I don't want to use any coordinate representation.
>
> The first thing that comes into mind would be to define a type "vector"
> and another type "scalar" and then define the usual rules.
>
> I'd like to execute a comand like
>
>     FunctionExpand[ expr , Assumptions->{{vA, vB, vC} "elem" vectors,
> {a,b,c} "elem" scalars}]
>
> Here's an example of a simple problem in which my question arises:
>
> Let the vectors vR1 and vR2 be defined in terms of four other vectors
> vA1, vA2, vN1, vN2 and two scalars a and b as follows:
>
> vR1 = vA1 + a vN1
> vR2 = vA2 + b vN2
>
> and consider the function
>
> U = (vR1 - vR2).(vR1 - vR2)
>
> The task is to find the minimum of U in terms of a combination of
> scalar product of the vectors vA1, vA2, vN1, vN2 and of the optimizing
> parameters a and b.
>
> Any help is greatly appreciated.
>
> Regards,
> Wolfgang

There are probably a few reasonable ways to go about this. One is to use a
special vector "head" and define rules for handling distribution involving
Plus, Times, and whatever else might be of relevance. I encapsulate your
vectors with vec[].

I find it convenient to move to a new function, myDot, for purposes of
intermediate computation. This avoids placing new behavior on Dot itself.

myDot[a_] /; Head[a] =!= vec := a
myDot[x___, y_Plus, z___] := Map[myDot[x, #, z] &, y]
myDot[x___, y_Dot, z___] := myDot[x, Apply[Sequence, y], z]
myDot[x___, y_Times, z___] := myDot[x, Apply[Sequence, y], z]
myDot[x___, y_myDot, z___] := myDot[x, Apply[Sequence, y], z]
myDot[] = 1;
myDot[x___, y_, z___] /; Head[y] =!= vec := y*myDot[x, z]

DotExpand[expr_] := (expr /. Dot -> myDot) /. myDot -> Dot

In[54]:= DotExpand[vec[vA].(b*vec[vB] + vec[vC])]

Out[54]= b vec[vA].vec[vB] + vec[vA].vec[vC]

The above myDot is an alteration of some code I cribbed from the section
"Some noncommutative algebraic manipulation" in the 1998 conference
notebook at:

http://library.wolfram.com/infocenter/Conferences/325/


Daniel Lichtblau
Wolfram Research





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