The isogeny theorem: $$\mathrm{Hom}_K(A,A') = \mathrm{Hom}_{G_K}(T_\ell(A),T_\ell(A')).$$
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The isogeny theorem: $$\mathrm{Hom}_K(A,A') = \mathrm{Hom}_{G_K}(T_\ell(A),T_\ell(A')).$$