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Subject: Re: [boost] different matrix library?
From: DE (satan66613_at_[hidden])
Date: 20090814 13:43:27
on 14.08.2009 at 21:23
Edward Grace wrote :
> Just to stick my oar in  there is also a subtle difference between
> a covector and a vector. This rarely seems to get a lookin when
> people implement linear algebra stuff. They may well be represented
> as a tuple in both cases but they interact differently. For example.
> covector*vector = scalar [inner product]
> vector*covector = tensor [outer product]
> Usually they are represented as columns (vectors) and rows (covectors).
> a) 1 2 3
> b) 1
> 2
> 3
> a*b = 14 (inner product)
> b*a =
> 1 2 3
> 2 4 6
> 3 6 9
> (outer product) [N.B. there are still only 6 independent components]
well if we consider a vector to be a column vector then inner product
will be
transpose(vector)*vector //covector*vector; actually here we
//get 1x1 matrix
and outer product
vector*transpose(vector) //vector*covector
since a vector is actually a matrix we can transpose it
> http://en.wikipedia.org/wiki/
> Einstein_notation#Common_operations_in_this_notation
> It would be great if these objects could be made to 'do the right
> thing', MATLAB does, mostly (it's MAT lab after all not TENS lab).
> Being able to write stuff like:
> vector w,u,v; // They are all (column) vectors.
> ...
> w = levi_civita*u*v;
> and get out the conventional cross product u x v when u and v have
> length 3, as a special case of exterior products, or,
> w = wedge(u,v); // Wedge product, function form
> w = u ^ v; // Wedge product operator form.
> Though I'm not sure it has the right precedence properties...
> would be pretty neat.
> I'll go away and hide now....
unfortunately i think in linalg domain (you think in tensor domain i
guess) so we may misanderstand each other
in the end i think it's possible to attach a mechanism to support
tensor style operations
by the way
> w = u ^ v;
it's terribly wrong to overload '^' in such a way
the following is much MUCH more better solution:
> w = wedge(u,v);
 Pavel
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