An extremal subharmonic function in non-archimedean potential theory

We define an analog of the Leja-Siciak-Zaharjuta subharmonic extremal function for a proper subset $E$ of the Berkovich projective line $P^1$ over a field with a non-archimedean absolute value, relative to a point $\zeta \not \in E$. When $E$ is a compact set with positive capacity, we prove that the upper semicontinuous regularization of this extremal function equals the Green function of $E$ relative to $\zeta$. As a separate result, we prove the Brelot-Cartan principle, under the additional assumption that the Berkovich topology is second countable.