is there a vector cross product function?

[Jordi Burguet Castell]

>  > Maybe its time for a gsl_tensor framework?
>
> There is a tensor directory on the tensor-1-3 branch in the gsl cvs
> repository on savannah.gnu.org, written by Jordi Burguet Castell.

Hello there.


> It is a tricky problem though, in my opinion.
>
> The approach there uses the correct type of struct, but I feel it
> needs to use arrays instead of varargs for passing the n-dimensional
> arguments in and out.

It is very nice to read that it is the correct type of struct, esp. coming 
from you, thanks!  The arrays-vs-varargs is one of the points where I feel 
more uneasy. I used varargs initially to get a cleaner syntax in user space, 
but maybe the trade is not worth it.


> The remaining unsolved issue is coming up with nice handling of views
> and mappings to matrices and vectors.  Without this the tensors can't
> interact with the rest of the linear algebra routines, which reduces
> their usefulness significantly in practice.  A major design decision
> is whether or not the tensor should allow physical sizes different
> from the tensor size itself (i.e. LDA vs N in BLAS/LAPACK
> terminology).  This has always been unclear to me.

I completely agree. My version doesn't handle views nor mappings at all, in 
part to simplify the code, and in part because I haven't figured out a way to 
do it. Tensor size is always equal to physical size, and the operations 
aren't optimized at all in the way that BLAS/LAPACK's are. The only strong 
point about them is that they work, and in a pretty general way.

Since last June or so, when I sent you my first draft, I have improved the GSL 
version of my C++-mini-tensor-library. I haven't commit-ed it to Savannah, 
though, as I wasn't sure of how to manage a branch and was afraid of screwing 
something up. Maybe I should commit anyway?

So, if you pardon my extra KB, I'm attaching the last version of the tensor 
directory I was working on for GSL (to "tar xzf" in the GSL/ directory, and 
change trivially the GSL/ Makefile.am and configure.ac), improved but not yet 
complete. Also, as reference, there is *again* the C++ version completely 
functional.

Before xmas I said I hoped to finish it before 2004. I didn't, but still 
hoping to finish it soon...  As usual, anybody willing to shortcircuit me and 
do it... blah blah.

Jordi

PS: In the spirit of the recently posted 
http://math-atlas.sourceforge.net/faq.html#utone , I must say that I use GSL 
quite a lot in my PhD work, and so do a lot of people in our experiment at 
Cern (Harp), and it is simply wonderful. Thanks a lot.
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