A (a) Show that if $u$ is superharmonic, then $u(x)\geq {\frac {1}{\alpha (n)r^{n}}}\int _{B(x,r)}u\,dy\quad {\text{ for all }}B(x,r)\subset U$ . (b) Prove that if $u$ is superharmonic, then $\min _{\bar {U}}u=\min _{\partial U}u.$ (c) Suppose $U$ is connected. Show that if there exists $x_{0}\in U$ such that $u(x_{0})=\min _{\bar {U}}u$ then $u$ is constant in $U$ . |

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