On the Amenability of Compact and Discrete Hypergroup Algebras

Let $K$ be a commutative compact hypergroup and $L^1(K)$ the hypergroup algebra. We show that $L^1(K)$ is amenable if and only if ${\pi }_K$, the Plancherel weight on the dual space $\widehat{K}$, is bounded. Furthermore, we show that if $K$ is an infinite discrete hypergroup and there exists $\alpha\in \widehat{K}$whichvanishesatinfinity,then$L^1(K)$ is not amenable. In particular, $L^1(K)$ fails to be even $\alpha$-left amenable if ${\pi }_K(\{\alpha \})=0$.