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We give a simple method to reduce the VBS-type states, usually expressed by the valence bonds, to the matrix-product representation. In this representation, peculiar properties of the VBS-type states are quite manifest. Using it, we investigate hidden order in the spin-S VBS model. The possibility that the Haldane phase for higher spin S is characterized by string order parameters is discussed. A picture of an approximate excitation is also presented.
Totsuka et al. (Mon,) studied this question.