| 摘 要： | In this paper,an approach is introduced to de-termine the stable interval of the constant term ofa characteristic equation by using the theory ofextended graphical representation of polynomials.Because the constant a_11 itself is not takeninto account and because this method is to get astable interval of a_n,not merely to make a stabi-lity test for a set of known coefficients,this me-thod of stability criteria has some advantages overthe others.The interval of a_n can be obtainedfrom the calculation of some algebraical expressionswhen n≤10,where n is the degree of the character-istic equation.It is very convenient to calculatefor the cases n=5 and n=6.When n≥11,thisinterval can be found only by the method of numeri-cal solutions.
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