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We comprehensively investigate the nontrivial states of an interacting Bose system in a cosine potential under the open boundary condition.Our results show that there exists a kind of stable localized state:edge gap solitons.We argue that the states originate from the eigenstates of independent edge parabolas.In particular,the edge gap solitons exhibit a nonzero topological-invariant behavior.The topological nature is due to the connection of the present model to the quantized adiabatic particle transport problem.In addition,the composition relations between the gap solitons and the extended states are also discussed.