Quadratically Hyponormal Recursively Generated Weighted Shifts Need Not Be Positively Quadratically Hyponormal |
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Authors: | Yiu T Poon Jasang Yoon |
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Institution: | (1) Department of Mathematics, Iowa State University, Ames, Iowa 50011, USA;(2) Department of Mathematics, The University of Texas-Pan American, Edinburg, Texas 78539, USA |
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Abstract: | We study a class of weighted shifts W
α defined by a recursively generated sequence α ≡ α0, … , α
m−2, (α
m−1, α
m
, α
m+1)∧ and characterize the difference between quadratic hyponormality and positive quadratic hyponormality. We show that a shift
in this class is positively quadratically hyponormal if and only if it is quadratically hyponormal and satisfies a finite
number of conditions. Using this characterization, we give a new proof of 12, Theorem 4.6], that is, for m = 2, W
α is quadratically hyponormal if and only if it is positively quadratically hyponormal. Also, we give some new conditions for
quadratic hyponormality of recursively generated weighted shift W
α (m ≥ 2). Finally, we give an example to show that for m ≥ 3, a quadratically hyponormal recursively generated weighted shift W
α need not be positively quadratically hyponormal. |
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Keywords: | Mathematics Subject Classification (2000)" target="_blank">Mathematics Subject Classification (2000) Primary 47B20 47B37 Secondary 47-04 |
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