杨氏定理:如果N个变量的函数f(X)有连续二阶偏微分,则交叉偏微分(混合偏微分)相等.杨氏定理Young`s Theorem 英文定义: Let f be a differentiable function of n variables. If each of the crosspartials f "ij and f "ji exists and is continuous at all points in some open set S of values of (x1, ..., xn) then f "ij(x1.., xn) = f "ji(x1.., xn) for all (x1.., xn) in S. 中文定义: 即: f "ij(x) = f "ji(x)