Let 
be a unital 
-algebra,
then an element 
is called an isometry if 
.
Isometry Element
This entry contributed by Mohammad Sal Moslehian
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References
Murphy, G. J. C-*-Algebras and Operator Theory. New York: Academic Press, 1990.Referenced on Wolfram|Alpha
Isometry ElementCite this as:
Moslehian, Mohammad Sal. "Isometry Element." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/IsometryElement.html