A note on numerical singular values of compositions with non-compact operators

Linear non-compact operators are difficult to study because they do not exist in the finite dimensional world. Recently, Mathé and Hofmann studied the singular values of the compact composition of the non-compact Hausdorff moment operator and the compact integral operator and found credible arguments, but no strict proof, that those singular values fall only slightly faster than those of the integral operator alone. However, the fact that numerically the singular values of the combined operator fall exponentially fast was not mentioned. In this note, we provide the missing numerical results and provide an explanation why the two seemingly contradicting results may both be true.

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