Let $\mathbf{F}=\mathbf{F}(x, y)=P(x, y) \mathbf{i}+Q(x, y) \mathbf{j}$ be a vector field, where the functions $P$ and $Q$ are continuous on some simply connected, open region $R .$ Suppose $\frac{\partial P}{\partial y}$ and $\frac{\partial Q}{\partial x}$ are also continuous on $R$. Then $\mathbf{F}(x, y)=P(x, y) \mathbf{i}+Q(x, y) \mathbf{j}$ is a conservative vector field on $R$ if and only if _________ .