Guardian maps and the generalized stability of parametrized families of matrices
L. Saydy, A. L. Tits, and E. H. Abed.
Mathematics of Control, Signals
and Systems, 3:345–371, 1990.
http://www.google.com.hk/url?sa=t&rct=j&q=Guardian+maps+and+the+generalized+stability+of&source=web&cd=1&ved=0CCoQFjAA&url=http%3A%2F%2Fwww.springerlink.com%2Findex%2F35246w6372647jj3.pdf&ei=kv_JTp6kM4vBiQf-z4TuDw&usg=AFQjCNHOt9eDFVopknXP5HOzUFoM62Qiyg&cad=rjt