% Generated by roxygen2: do not edit by hand
% Please edit documentation in R/centrality.R
\name{arpack_defaults}
\alias{arpack_defaults}
\alias{arpack}
\alias{arpack-options}
\alias{arpack.unpack.complex}
\title{ARPACK eigenvector calculation}
\usage{
arpack_defaults()

arpack(
  func,
  extra = NULL,
  sym = FALSE,
  options = arpack_defaults(),
  env = parent.frame(),
  complex = !sym
)
}
\arguments{
\item{func}{The function to perform the matrix-vector multiplication. ARPACK
requires to perform these by the user. The function gets the vector \eqn{x}
as the first argument, and it should return \eqn{Ax}, where \eqn{A} is the
\dQuote{input matrix}. (The input matrix is never given explicitly.) The
second argument is \code{extra}.}

\item{extra}{Extra argument to supply to \code{func}.}

\item{sym}{Logical scalar, whether the input matrix is symmetric. Always
supply \code{TRUE} here if it is, since it can speed up the computation.}

\item{options}{Options to ARPACK, a named list to overwrite some of the
default option values. See details below.}

\item{env}{The environment in which \code{func} will be evaluated.}

\item{complex}{Whether to convert the eigenvectors returned by ARPACK into R
complex vectors. By default this is not done for symmetric problems (these
only have real eigenvectors/values), but only non-symmetric ones. If you
have a non-symmetric problem, but you're sure that the results will be real,
then supply \code{FALSE} here.}
}
\value{
A named list with the following members: \item{values}{Numeric
vector, the desired eigenvalues.} \item{vectors}{Numeric matrix, the desired
eigenvectors as columns. If \code{complex=TRUE} (the default for
non-symmetric problems), then the matrix is complex.} \item{options}{A named
list with the supplied \code{options} and some information about the
performed calculation, including an ARPACK exit code. See the details above.
}
}
\description{
Interface to the ARPACK library for calculating eigenvectors of sparse
matrices
}
\details{
ARPACK is a library for solving large scale eigenvalue problems.  The
package is designed to compute a few eigenvalues and corresponding
eigenvectors of a general \eqn{n} by \eqn{n} matrix \eqn{A}. It is most
appropriate for large sparse or structured matrices \eqn{A} where structured
means that a matrix-vector product \code{w <- Av} requires order \eqn{n}
rather than the usual order \eqn{n^2} floating point operations.

This function is an interface to ARPACK. igraph does not contain all ARPACK
routines, only the ones dealing with symmetric and non-symmetric eigenvalue
problems using double precision real numbers.

The eigenvalue calculation in ARPACK (in the simplest case) involves the
calculation of the \eqn{Av} product where \eqn{A} is the matrix we work with
and \eqn{v} is an arbitrary vector. The function supplied in the \code{fun}
argument is expected to perform this product. If the product can be done
efficiently, e.g. if the matrix is sparse, then \code{arpack()} is usually
able to calculate the eigenvalues very quickly.

The \code{options} argument specifies what kind of calculation to perform.
It is a list with the following members, they correspond directly to ARPACK
parameters. On input it has the following fields: \describe{
\item{bmat}{Character constant, possible values: \sQuote{\code{I}}, standard
eigenvalue problem, \eqn{Ax=\lambda x}{A*x=lambda*x}; and \sQuote{\code{G}},
generalized eigenvalue problem, \eqn{Ax=\lambda B x}{A*x=lambda B*x}.
Currently only \sQuote{\code{I}} is supported.} \item{n}{Numeric scalar. The
dimension of the eigenproblem. You only need to set this if you call
\code{\link[=arpack]{arpack()}} directly. (I.e. not needed for
\code{\link[=eigen_centrality]{eigen_centrality()}}, \code{\link[=page_rank]{page_rank()}}, etc.)}
\item{which}{Specify which eigenvalues/vectors to compute, character
constant with exactly two characters.

Possible values for symmetric input matrices: \describe{
\item{"LA"}{Compute \code{nev} largest (algebraic) eigenvalues.}
\item{"SA"}{Compute \code{nev} smallest (algebraic)
eigenvalues.} \item{"LM"}{Compute \code{nev} largest (in
magnitude) eigenvalues.} \item{"SM"}{Compute \code{nev} smallest
(in magnitude) eigenvalues.} \item{"BE"}{Compute \code{nev}
eigenvalues, half from each end of the spectrum. When \code{nev} is odd,
compute one more from the high end than from the low end.} }

Possible values for non-symmetric input matrices: \describe{
\item{"LM"}{Compute \code{nev} eigenvalues of largest
magnitude.} \item{"SM"}{Compute \code{nev} eigenvalues of
smallest magnitude.} \item{"LR"}{Compute \code{nev} eigenvalues
of largest real part.} \item{"SR"}{Compute \code{nev}
eigenvalues of smallest real part.} \item{"LI"}{Compute
\code{nev} eigenvalues of largest imaginary part.}
\item{"SI"}{Compute \code{nev} eigenvalues of smallest imaginary
part.} }

This parameter is sometimes overwritten by the various functions, e.g.
\code{\link[=page_rank]{page_rank()}} always sets \sQuote{\code{LM}}.  }
\item{nev}{Numeric scalar. The number of eigenvalues to be computed.}
\item{tol}{Numeric scalar. Stopping criterion: the relative accuracy of the
Ritz value is considered acceptable if its error is less than \code{tol}
times its estimated value. If this is set to zero then machine precision is
used.} \item{ncv}{Number of Lanczos vectors to be generated.}
\item{ldv}{Numberic scalar. It should be set to zero in the current
implementation.} \item{ishift}{Either zero or one. If zero then the shifts
are provided by the user via reverse communication. If one then exact shifts
with respect to the reduced tridiagonal matrix \eqn{T}.  Please always set
this to one.} \item{maxiter}{Maximum number of Arnoldi update iterations
allowed. } \item{nb}{Blocksize to be used in the recurrence. Please always
leave this on the default value, one.} \item{mode}{The type of the
eigenproblem to be solved.  Possible values if the input matrix is
symmetric: \describe{ \item{1}{\eqn{Ax=\lambda x}{A*x=lambda*x}, \eqn{A} is
symmetric.} \item{2}{\eqn{Ax=\lambda Mx}{A*x=lambda*M*x}, \eqn{A} is
symmetric, \eqn{M} is symmetric positive definite.} \item{3}{\eqn{Kx=\lambda
Mx}{K*x=lambda*M*x}, \eqn{K} is symmetric, \eqn{M} is symmetric positive
semi-definite.} \item{4}{\eqn{Kx=\lambda KGx}{K*x=lambda*KG*x}, \eqn{K} is
symmetric positive semi-definite, \eqn{KG} is symmetric indefinite.}
\item{5}{\eqn{Ax=\lambda Mx}{A*x=lambda*M*x}, \eqn{A} is symmetric, \eqn{M}
is symmetric positive semi-definite. (Cayley transformed mode.)} } Please
note that only \code{mode==1} was tested and other values might not work
properly.

Possible values if the input matrix is not symmetric: \describe{
\item{1}{\eqn{Ax=\lambda x}{A*x=lambda*x}.} \item{2}{\eqn{Ax=\lambda
Mx}{A*x=lambda*M*x}, \eqn{M} is symmetric positive definite.}
\item{3}{\eqn{Ax=\lambda Mx}{A*x=lambda*M*x}, \eqn{M} is symmetric
semi-definite.} \item{4}{\eqn{Ax=\lambda Mx}{A*x=lambda*M*x}, \eqn{M} is
symmetric semi-definite.} } Please note that only \code{mode==1} was tested
and other values might not work properly.  } \item{start}{Not used
currently. Later it be used to set a starting vector.} \item{sigma}{Not used
currently.} \item{sigmai}{Not use currently.}

On output the following additional fields are added: \describe{
\item{info}{Error flag of ARPACK. Possible values: \describe{
\item{0}{Normal exit.} \item{1}{Maximum number of iterations taken.}
\item{3}{No shifts could be applied during a cycle of the Implicitly
restarted Arnoldi iteration. One possibility is to increase the size of
\code{ncv} relative to \code{nev}.} }

ARPACK can return more error conditions than these, but they are converted
to regular igraph errors.  } \item{iter}{Number of Arnoldi iterations
taken.} \item{nconv}{Number of \dQuote{converged} Ritz values. This
represents the number of Ritz values that satisfy the convergence critetion.
} \item{numop}{Total number of matrix-vector multiplications.}
\item{numopb}{Not used currently.} \item{numreo}{Total number of steps of
re-orthogonalization.} } } Please see the ARPACK documentation for
additional details.
}
\examples{

# Identity matrix
f <- function(x, extra = NULL) x
arpack(f, options = list(n = 10, nev = 2, ncv = 4), sym = TRUE)

# Graph laplacian of a star graph (undirected), n>=2
# Note that this is a linear operation
f <- function(x, extra = NULL) {
  y <- x
  y[1] <- (length(x) - 1) * x[1] - sum(x[-1])
  for (i in 2:length(x)) {
    y[i] <- x[i] - x[1]
  }
  y
}

arpack(f, options = list(n = 10, nev = 1, ncv = 3), sym = TRUE)

# double check
eigen(laplacian_matrix(make_star(10, mode = "undirected")))

## First three eigenvalues of the adjacency matrix of a graph
## We need the 'Matrix' package for this
if (require(Matrix)) {
  set.seed(42)
  g <- sample_gnp(1000, 5 / 1000)
  M <- as_adjacency_matrix(g, sparse = TRUE)
  f2 <- function(x, extra = NULL) {
    cat(".")
    as.vector(M \%*\% x)
  }
  baev <- arpack(f2, sym = TRUE, options = list(
    n = vcount(g), nev = 3, ncv = 8,
    which = "LM", maxiter = 2000
  ))
}
}
\references{
D.C. Sorensen, Implicit Application of Polynomial Filters in a
k-Step Arnoldi Method. \emph{SIAM J. Matr. Anal. Apps.}, 13 (1992), pp
357-385.

R.B. Lehoucq, Analysis and Implementation of an Implicitly Restarted Arnoldi
Iteration. \emph{Rice University Technical Report} TR95-13, Department of
Computational and Applied Mathematics.

B.N. Parlett & Y. Saad, Complex Shift and Invert Strategies for Real
Matrices. \emph{Linear Algebra and its Applications}, vol 88/89, pp 575-595,
(1987).
}
\seealso{
\code{\link[=eigen_centrality]{eigen_centrality()}}, \code{\link[=page_rank]{page_rank()}},
\code{\link[=hub_score]{hub_score()}}, \code{\link[=cluster_leading_eigen]{cluster_leading_eigen()}} are some of the
functions in igraph that use ARPACK.
}
\author{
Rich Lehoucq, Kristi Maschhoff, Danny Sorensen, Chao Yang for
ARPACK, Gabor Csardi \email{csardi.gabor@gmail.com} for the R interface.
}
\concept{arpack}
\keyword{graphs}