The dynamics of collisionless galaxy can be described by the Vlasov-Poisson system. By the Jeans theorem, all the spherically symmetric steady galaxy models are given by a distribution of {Phi}(E,L), where E is the particle energy and L the angular momentum. In a celebrated Doremus-Feix-Baumann Theorem, the galaxy model {Phi}(E,L) is stable if the distribution {Phi} is monotonically decreasing with respect to the particle energy E. On the other hand, the stability of {Phi}(E,L) remains largely open otherwise. Based on a recent abstract instability criterion of Guo-Lin, we constuct examples of unstable galaxy models of f(E,L) and f(E) in which f fails to be monotone in E.
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