[R] help with PCA and classical scaling

[Domenico Cozzetto]

Dear all,
I have a matrix of dissimilarities and I'd like to get a 2d embedding. I'm
not an expert of these techniques, so I'm a bit confused... Furthermore I
was not able to find on the web any satisfactory tutorial. So even though
this may be not the most appropriate place to discuss about this issues, I'd
be very grateful to those who will reply.

My first question is: Do I need an initial embedding of my data before
applying PCA through the methods princomp or prcomp in the stats package?

What does it mean that PCA and classical scaling are equivalent? And where
can I find a proof of this fact?

If this is true, I should get the same results by applying the methods
prcomp() or cmdscale() in stats. If it may help, use the dissimilarities in
the attached "diss.tab" file.

Thank you very much in advance for your attention.
Domenico