Any VAR(p)-process can be written in a first-order vector
autoregressive form: the companion form. A VAR(p)-process is stable,
if its reverse characteristic polynomial:
$$
\det(I_K - A_1 z - \cdots - A_p z^p) \neq 0 \; \hbox{for} \; |z| \le 1
\; ,
$$
has no roots in or on the complex circle. This is equivalent to the
condition that all eigenvalues of the companion matrix \(A\) have
modulus less than 1. The function roots(), does compute the
eigen values of the companion matrix \(A\) and returns by default
their moduli.