Division of Applied Mathematics, Department of Electrical and Electronic Engineering, Faculty of Engineering, Ehime University, Matsuyama, 790-8577, Japan (e-mail: inoue@ee.ehime-u.ac.jp)
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In [1], Lemma 6.6 is wrong in the case d_U=d_V=2. The proof of this lemma includes an uncorrectable error. Thus, the proof of Theorem 2 is invalid.