| 摘 要： | In this paper we study the degree of approximation by superpositions of a sigmoidal function.We mainlyconsider the univariate case.If f is a continuous function,we prove that for any bounded sigmoidalfunction σ,d_(n,σ)(f)≤‖σ‖ω(f,1/(n+1)).For the Heaviside function H(x),we prove that d_(n,H)(f)≤ω(f,1/(2(n+1))).If f is a continuous funnction of bounded variation,we prove that d_(n,σ)(f)≤‖σ‖/(n+1)V(f)and d_(n,H)(f)≤1/(2(n+1))V(f).For he Heaviside function,the coefficient 1 and the approximation orders are the bestpossible.We compare these results with the classical Jackson and Bernstein theorems,and make some conjec-tures for further study.
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