A unilateral shift is a weighted shift 
for which 
for all 
.
Unilateral Shift
See also
Weighted ShiftThis entry contributed by Mohammad Sal Moslehian
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Moslehian, Mohammad Sal. "Unilateral Shift." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/UnilateralShift.html