A linear bounded operator A in a complex Hilbert space H is called a two-isometry if A~*2A~2 - 2A~*A + I = 0. In particular, the class of two-isometries contains conventional isometries. It is shown that in the finite-dimensional case, the notion of two-isometry has no new content, that is, two-isometries of a finite-dimensional unitary space are conventional unitary operators.
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机译：如果A〜* 2A〜2-2A〜* A + I = 0，则复希尔伯特空间H中的线性有界算子A称为二等距。特别地，二等距的类别包含常规等距。结果表明，在有限维情况下，二维等距的概念没有新的内容，也就是说，有限维unit空间的两个等距是常规的operators算子。
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