We present necessary and sufficient conditions such that the output fr
om the Teager-Kaiser energy operator [s(t) = ds(t)/dt] PSI(c)[s(t)] =
s2(t) - s(t)s(t) (1) for continuous-time signals s(t) and the output f
rom the corresponding discrete-time energy operator PSI(d)[s(n)] = s2(
n) - s(n+1)s(n-1) (2) be non-negative everywhere.