With the default settings, a square \(m\times m\) matrix is
returned, where \(m\) is the length of v
. If ncol>
\(m\),
the vector is amended with 0's. It is an error for ncol
to be
smaller than \(m\).
Argument nrow
may be used to get a rectangular matrix, although
the term "companion" is normally used only for square matrices.