Topological insulator(TI) is a phase of matter discovered recently. Kane and Mele proposed this phase is distinguished from the ordinary band insulator by a Z2 topological invariant.2 Several authors have try to related this Z2 invariant to Chern numbers. Roy find a way to calculate Z2 by Chern Number of one of the two degenerate Bands or one-band Chern number(OBChN). However, he give no concrete concrete proof of the equivalence of his Z2 and the Z2 in ref beside \from the topological considerations of K theory". So the importance of OBChN hasn't been recognized by the community. In this letter we prove OBChN determines the Z2 in ref. Then we illustrate OBChN is not only an useful tool to identify TI but also a natural criterion to classify topological property of all time-reversal invariant band insulators. More importantly we find a field in three dimensional TI can be identified with magnetic field with magnetic monopole
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