Re: manual chapter about diagonal and permutation matrices

[Søren Hauberg]

tir, 24 02 2009 kl. 09:02 +0100, skrev Jaroslav Hajek:
> > 2) You reference Wikipedia twice. In general we try to avoid linking to
> >   online stuff since the manual is also printed. If we should link to
> >   online stuff then I think it requires some discussion. In this
> >   specific case, I don't really think the links add much information,
> >   so I think they should simply be removed.
> 
> Since I only provide basic definitions of what diagonal and
> permutation matrix is, I wanted to refer the reader to more
> information. I regard Wikipedia as reliable online source and I don't
> think references to internet sources are becoming increasingly common
> even in printed papers. Note that Wikipedia is easily accessible
> worldwide, which you can't say for many textbooks.

I understand (and mostly agree) with all this. My comment was about
consistency in the sense that we don't really link to online sites in
many other places in the manual. If we have a link to Wikipedia about
diagonal matrices, then why not have one about matrix multiplication? If
we have such links, when do we add them, and which sites can we refer
to?

> > 3) I don't really understand the text that describes how to create
> >   permutation matrices. I think it would be nice to have a more
> >   explicit example.
> 
> OK, can you clarify this? The text shows the exact code for 3 ways of
> creating a permutation matrix - by indexing the (diagonal) identity
> matrix. I'm not sure how "more explicit" it could be.

So, my confusions comes from simple things. For example, it is not clear
to me what a "permutation vector" is. I'm guessing that [3, 2, 1] is a
permutation vector that reverses the order of matrix rows. This is,
however, not clear from the text.

When I ask for something more explicit I mean

"
To create the permutation matrix

        | 1, 0, 0 |
    P = | 0, 0, 1 |
        | 0, 1, 0 |

write

    P = eye (3) (:, [1, 3, 2])
"

or something like that. I think it is important to actually show
contents of the permutation matrices to make sure there are no
misunderstanding.

Soren