Let F/Q be a CM field where p splits completely andr : Gal(Q/F ) → GL3(Fp) a continuous modular Galois representation. Assume thatr is non-ordinary and non-split reducible (niveau 2) at a place w above p. We show that the isomorphism class ofr| Gal(F w /Fw ) is determined by the GL3(Fw)-action on the space of mod p algebraic automorphic forms using the refined Hecke action of Herzig, Le and Morra [Compos. Math. 153 (2017) 2215-2286. We also give a nearly optimal weight elimination result for niveau 2 Galois representations compatible with the explicit conjectures of Gee, Herzig and Savitt [J. Eur. Math. Soc., to appear] and Herzig [Duke Math. J. 149 (2009) 37-116]. Moreover, we prove the modularity of certain Serre weights, in particular, when the Fontaine-Laffaille invariant takes special value ∞, our methods establish the modularity of a certain shadow weight.
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