# Re: [R] highest eigenvalues of a matrix

From: Katharine Mullen <kate_at_few.vu.nl>
Date: Thu, 19 Jun 2008 20:08:04 +0200 (CEST)

> >>>>> "KateM" == Katharine Mullen <kate@few.vu.nl>
> >>>>> on Thu, 19 Jun 2008 11:50:22 +0200 (CEST) writes:
>
> KateM> On Thu, 19 Jun 2008, Simon Wood wrote:
> >> >
> >> > I happily use eigen() to compute the eigenvalues and
> >> eigenvectors of > a fairly large matrix (200x200, say),
> >> but it seems over-killed as its > rank is limited to
> >> typically 2 or 3. I sort of remember being taught > that
> >> numerical techniques can find iteratively decreasing
> >> eigenvalues > and corresponding orthogonal eigenvectors,
> >> which would provide a nice > alternative (once I have the
> >> first 3, say, I stop the search).
> >>
> >> Lanczos iteration will do this efficiently (see
> >> e.g. Golub & van Loan "Matrix Computations"), but I don't
> >> think that there are any such routines built into R or
> >> LAPACK (although I haven't checked the latest LAPACK
> >> release). When I looked it seemed that the LAPACK options
> >> that allow you to select eigen-values/vectors still
> >> depend on an initial O(n^3) decomposition of the matrix,
> >> rather than the O(n^2) that a Lanczos based method would
> >> require.
> >>
>
> KateM> ARPACK (http://www.caam.rice.edu/software/ARPACK/) uses a
> KateM> Lanczos method for symmetric matrics; otherwise it uses an
> KateM> Arnoldi iteration. Development of an R interface to ARPACK
> KateM> would be a nice project (but unfortunately one I don't have
> KateM> time for for a while). Maybe one of the maintainers of a
> KateM> package for sparse matrices would be interested.
>
> Yes, thank you, Kate, we have been.
> The nice thing in ARPACK is its concept of "reverse
> communication", such that we should be able to use it both
> for sparse and for dense matrices.
> We would only have to provide a "function" for computing
> "A %*% x" and hence could use different ones, depending on the
> class of A.... all in theory at least.
>
> One problem I see with the with that code is that its README file contains
>
> *** NOTE *** Unless the LAPACK library on your system is version 2.0,
> we strongly recommend that you install the LAPACK routines provided with
> ARPACK. Note that the current LAPACK release is version 3.0; if you are
> not sure which version of LAPACK is installed, pleaase compile and link
> to the subset of LAPACK included with ARPACK.
>
> i.e. you need to use old/outdated Lapack with ARPACK instead of
> the current Lapack . Something that sheds a bad light to ARPACK
> in my view.
>

Dear Martin,

I agree this is not ideal. I wonder why it has not been kept up to date with LAPACK and how broken it is with version 3.1.1. (but can't experiment now).

> In the mean time (I've suffered from a computer crash and other
> interruptions) I see Gabor has already mentioned the following
>
> Then, we have seen that the 'igraph' package als uses a port of
> (part of) ARPACK as part of the C++ 'igraphlib' library.
>
>
>
> >> My `mgcv' package (see cran) uses Lanczos iteration for
> >> setting up low rank bases for smoothing. The source code
> >> is in mgcv/src/matrix.c:lanczos_spd, but I'm afraid that
> >> there is no direct R interface, although it would not be
> >> too hard to write a suitable wrapper. It requires the
> >> matrix to be symmetric.
> >>
> >> > Looking at the R source code for eigen and some posts
> >> on this list, > it seems that the function uses a LAPACK
> >> routine, but obviously all > the options are not
> >> available through the R wrapper. I'm not > experienced
> >> enough to try and make my own interface with Fortran >
> >> code, so here are two questions:
> >> >
> >> > - is this option (choosing a desired number of
> >> eigenvectors) already > implemented in some function /
> >> package that I missed? --- In the symmetric case you can
> >> use `svd' which lets you select (although you'd need to
> >> fix up the signs of the singular values to get
> >> eigen-values if the matrix is not +ve definite). But this
> >> answer is pretty useless as it will be slower than using
> >> `eigen' and getting the full decomposition.
> >>
> >> Of course if you know that your matrix is low rank
> >> because it's a product of non-square matrices then
> >> there's usually some way of getting at the
> >> eigen-decomposition efficiently. E.g. if A=B'B where B is
> >> 3 by 1000, then the cost can easily be kept down to
> >> O(1000^2) in R...
> >>
> >> best, Simon
> >>
> >> > - is the "range of indices" option in DSYEVR.f <
> >> http:// > www.netlib.org/lapack/double/dsyevr.f > what I
> >> think, the indices of > the desired eigenvalues ordered
> >> from the highest to lowest?
> >> >
>
> KateM> dsyevr.f will work with symmetric real matrices only. When the range
> KateM> argument of dysevr is set to 'I', arguments il and iu
> KateM> seem to specify the range of eigenvalue indices you
> KateM> want in ascending order (lowest to highest, not
> KateM> highest to lowest). If you look at
> KateM> https://svn.r-project.org/R/trunk/src/modules/lapack/Lapack.c
> KateM> you see that range is always set to 'A'.
>
> yes, but do you agree that using 'I' (and a range) does not
> really help much, since the factorization used is the "full" one
> anyway ?
>

Eigenvalues and eigenvectors can be
* selected by specifying either a range of values or a range of * indices for the desired eigenvalues.

with _selected_ magically replaced by _computed_. Thanks.

>
> >> > Many thanks in advance for any piece of advice,
>
> >> > Sincerely,
> >> >
> >> > Baptiste
> >> >
> >> > dummy example if needed:
> >> >
> >> > test <- matrix(c(1, 2, 0, 4, 5, 6, 1.00001, 2, 0),
> >> ncol=3) > eigen(test)
> >> >
> >> >
> >> >
> >> >
> >> > _____________________________
> >> >
> >> >
> >> > Physics Department > University of Exeter > Stocker
> >> Road, > Exeter, Devon, > EX4 4QL, UK
> >> >
> >> > Phone: +44 1392 264187
> >> >
> >> > http://newton.ex.ac.uk/research/emag >
> >> http://projects.ex.ac.uk/atto
> [............]
>
>
> >> --
> >> > Simon Wood, Mathematical Sciences, University of Bath,
> >> Bath, BA2 7AY UK > +44 1225 386603
> >> www.maths.bath.ac.uk/~sw283
>

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