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In the fuzzy evaluation process of the power quality, different membership function scan lead to different results, which affect the credibility of the evaluation. In order to avoid subjectivity of choosing membership function, a combined membership function based on the Variance-covariance optimized combination method is proposed. Firstly, five typical member-ship function is selected after the deeply analysis of the power quality data. Secondly, weights of each function are calculated by the variance-covariance optimized combination method, and then a combined membership function of general applicability is constructed according to the weights. Thirdly, the fuzzy evaluation is performed by using this combined membership function. Finally, an application of this combined membership function in the fuzzy evaluation of power quality of a power supply bureau shows that this function can improve credibility of three valuation result.

We present the time-varying critical fertility and analysis the stability of the system at the critical value. Then we get the sufficient conditions about the system stability, asymptotically stability and not. Keywords: Population development system; mortality rates; Critical fertility rate of women; stability