Green line

An orthogonal trajectory of a family of level surfaces $G_y(x)=r$, $0\leq r<+\infty$, where $G_y(x)=G(x,y)$ is the Green function (of the Dirichlet problem for the Laplace equation) for a domain $D$ in a Euclidean space $\mathbf R^n$, $n\geq2$, with a given pole $y\in D$. In other words, Green lines are integral curves in the gradient field $\operatorname{grad}G_y(x)$. Generalizations also exist [2].

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