An isometry element of a unital 
-algebra 
is an element 
such that 
.
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:
Weisstein, Eric W., with contributions by Mohammad Sal Moslehian. "Isometry Element." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/IsometryElement.html