A bounded operator 
on a Hilbert space 
is called essentially unitary if 
and 
are compact operators.
Essentially Unitary
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
Essentially UnitaryCite this as:
Moslehian, Mohammad Sal. "Essentially Unitary." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/EssentiallyUnitary.html