Anders Hansen (Cambridge)

The Solvability Complexity Index and Approximations of Spectra of Operators

In this talk we will discuss the following long standing and fundamental problem: Given an operator on a separable Hilbert space (with an orthonormal basis), can one compute/construct its spectrum from its matrix elements. As we want such a construction to be useful in application (i.e. implementable on a computer), we restrict ourselves to only allowing the use of arithmetic operations and radicals of the matrix elements and taking limits. We will give an affirmative answer to the question, and also introduce a classification tool for the complexity of different computational spectral problems, namely, the Solvability Complexity Index.