ABSTRACT
Powell (1997) can be regarded as an operator A from S to and this operator possesses only the first three of the properties (3.6).
The polynomial reproduction and ellipticity conditions imply nonsingularity. Specifically, given any nonzero s E 5, we ask if it is in IIm. If the answer is affirmative, then polynomial reproduction provides A s = s ^ 0. Otherwise, ellipticity gives (s,v4s)>0, which shows A s ^ O as required.
