1. Root-locus Equations of the Fourth Degree†
2. CHANG , S. S. L. , 1961 ,Synthesis of Optimum Control Systems( New York : McGraw-Hill Book Co. ), p. 29 .
3. CHESTNUT , H. , and MAYER , R. W. , 1957 ,Servomechanisms and Regulating System Design, Vol. 1 ( New York : John Wiley & Sons, Inc. ), p. 413 .