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We point out that the proof of one of the theorems presented in the above paper by Elia et al. (see ibid. vol.42 (1997)) is invalid and provide a correct proof. The flawed proof is long and complex, and incorrectly tries to make use of duality results via a version of the Lagrange multiplier theorem. The proof we presented, based on an elementary convexity argument, is very simple.
Date of Publication: Nov. 2000