matStig {expm}

Stig's "infamous" Example Matrix

Description

Stig Mortensen wrote on Oct 22, 2007 to the authors of the Matrix package with subject “Strange result from expm”. There, he presented the following 8 \times 8 matrix for which the Matrix expm() gave a “strange” result. As we later researched, the result indeed was wrong: the correct entries were wrongly permuted. The reason has been in the underlying source code in Octave from which it had been ported to Matrix.

Usage

data(matStig)

Author(s)

Martin Maechler

Examples

data(matStig)

as(matStig, "sparseMatrix") # since that prints more nicely.

## For more compact printing:
op <- options(digits = 4)

E1 <- expm(matStig, "Ward77", preconditioning="buggy") # the wrong result
as(E1, "sparseMatrix")
str(E2 <- expm(matStig, "Pade"))# the correct one (has "accuracy" attribute)
as(E2, "sparseMatrix")
attr(E2,"accuracy") <- NULL   # don't want it below
E3 <- expm(matStig, "R_Eigen")  # even that is fine here
all.equal(E1,E2) # not at all equal (rel.difference >~= 1.)
stopifnot(all.equal(E3,E2)) # ==

##________ The "proof" that "Ward77" is wrong _________
M <- matStig
Et1 <- expm(t(M), "Ward77", precond= "buggy")
Et2 <- expm(t(M), "Pade"); attr(Et2,"accuracy") <- NULL
all.equal(Et1, t(E1)) # completely different (rel.diff ~ 1.7 (platform dep.))
stopifnot(all.equal(Et2, t(E2))) # the same (up to tolerance)

options(op)