摘要
设Ｆ（ｘ，ｙ）是二元连续型分布函数，ｆ（ｘ，ｙ）是其密度函数，本文的目的是给出公式α＾２Ｆ（ｘ，ｙ）／αｘαｙ＝ｆ（ｘ，ｙ）成立的一个充分条件。
Suppose F(x,y) is a two-dimensional continuous distribution function, f(x,y) is its probability density function. The purpose of this paper is to give a sufficient condition of the formulaTheorem suppose f(x,y) is the probability density of two-dimensional continuous distribution F(x,y),if(a) f(x,y) is approximately continuous at the point (x0,y0).(b) There are a neighborhood G at the point (x0,y0) and a constant M > 0 such that|f(x,y)|<Mfor every (x, y)∈ G.(c) F''xy(x, y) exists at the point (x0,y0),thenF''xy(x0,y0) = f(x0,y0)
