翻译数学物理方法4. We use the following result from the theory of or

翻译数学物理方法
4. We use the following result from the theory of ordinary differential equations (see, for example, Ding and Li [2], Chapter 2, Theorem 1).
Theorem 1. Suppose function P (x, y) and Q(x, y) are continuous on U = (α, β) × (γ, δ), and they have continuous partial derivatives ∂P/∂y and ∂Q/∂x . Then the 1-form ω = P (x, y)dx + Q(x, y)dy is exact if and only if ∂P/∂y = ∂Q/∂x on U . Moreover, the 0-form whose differential is ω can be represented as
x y
∫ P (ξ, y)dξ + ∫ Q(x0, η)dη +C, where C is a constant.
x0 y0

已提高悬赏至200分，认真点翻。明天就考试了，求大神助攻！！^_^!!
PS：或者说下这条是什么定律