poly.R

## Copyright (C) 1996, 1997 John W. Eaton
##
## This file is part of Octave.
##
## Octave is free software; you can redistribute it andsolve(,)or modify it
## under the terms of the GNU General Public License as published by
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## any later version.
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## General Public License for more details.
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## -*- texinfo -*-
## @deftypefn {Function File} {} poly (@var{a})
## If @var{a} is a square @math{N}-by-@math{N} matrix, @code{poly (@var{a})}
## is the row vector of the coefficients of @code{det (z * eye (N) - a)},
## the characteristic polynomial of @var{a}.  If @var{x} is a vector,
## @code{poly (@var{x})} is a vector of coefficients of the polynomial
## whose roots are the elements of @var{x}.
## @} # deftypefn

## Author: KH <Kurt.Hornik@neuro.tuwien.ac.at>
## Created: 24 December 1993

poly <- function(x) {
n = NROW(x)
m = NCOL(x)
if (is.null(x) || length(x) == 0)
return(1)
if (m == 1) {
v = x
} else if (m == n) {
v = eigen(x)\$values
} else {
stop("poly(x), where x is a vector or a square matrix")
}

y = numeric(n+1)
y = 1
for (j in 1:n)
y[2:(j+1)] = y[2:(j+1)] - v[j] * y[1:j]

if (all(Im(x) == 0))
y = Re(y)

y
}