Prairie Mathematics Colloquium: Raphaël Clouâtre

Let B be a normed space and let A be a subspace. When does a continuous linear functional on A admit a unique norm-preserving extension to B? If A and B are C*-algebras and the functional is a so-called pure state, this question was at the heart of a long-standing conjecture of Kadison and Singer from 1959, which was eventually verified in 2015 by Marcus, Spielman and Srivastava. In this talk I will explore the corresponding problem for pure states on subspaces of C*-algebras. I will explain how this is non-trivial even for familiar choices of B, such as the continuous functions on some compact space, or the n x n matrices. I will attempt to clarify this issue using a non-commutative counterpart to the classical notion of peak point from function theory.

This event is part of the Prairie Mathematics Colloquium.