 Radial integral potential for a curl-free field

For a continuously differentiable curl-free vector field $F$ on a domain star-shaped about the origin, define $\Phi$ by the displayed integral. Differentiation and ${\partial }_i{F}_j={\partial }_j{F}_i$ give ${\partial }_i\Phi ={\int }_0^1[F_i(tx)+t{x}_j{\partial }_j{F}_i(tx)]dt={\int }_0^1{\partial }_t[t{F}_i(tx)]dt=F_i(x)$. Thus $F=\nabla \Phi$. To adopt the sign convention $F=-\nabla \varphi$, take $\varphi =-\Phi$. The star-shaped-domain hypothesis ensures every segment used in the construction lies in the domain; curl-free fields need not have global potentials on arbitrary domains.