Consider the difference equation system yt=b+Ayt-i; where yt is an nXl variable vector, b is an nXl constant vector and A is an nXn constant matrix. The fundamental necessary and sufficient stability condition for the system is /x(^)< 1, where [J.(A) is the modulus of the characteristic root of A of largest modulus. That is, IJ.(A) is the spectral radius of A, so-called because all roots of A lie on or within a circle of radius ix(A) in the complex plane. There exists a class of scalar real-valued functions of matrices called matrix norms; and any matrix norm/(y4) has the property iJ^(A)