Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2015/iii/paper-5/2/4/solution

No. On the complex l2 sequence space, let . This is a bounded self-adjoint operator. At , , so zero is not an eigenvalue (an eigenvector must be nonzero). But has no preimage in : its only formal preimage is the constant-one sequence. Thus is not surjective and
Alternatively excludes a bounded inverse. This example separates the spectrum of a bounded operator from its point spectrum.

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