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Crouzeix's Conjecture

Wednesday, January 27, 2016 - 9:00am - 9:50am
Keller 3-180
Michael Overton (New York University)
Crouzeix's conjecture is among the most intriguing developments in matrix theory in recent years. Made in 2004 by Michel Crouzeix, it postulates that, for any polynomial p and any matrix A, p(A) is less than or equal to 2 max(p(z): z in W(A)), where the norm is the 2-norm and W(A) is the field of values (numerical range) of A, that is the set of points attained by v*Av for some vector v of unit length. Remarkably, Crouzeix proved in 2007 that the inequality above holds if 2 is replaced by 11.08. Furthermore, it is known that the conjecture holds in a number of special cases, including n=2. We use nonsmooth optimization to investigate the conjecture numerically by attempting to minimize the “Crouzeix ratio”, defined as the quotient with numerator the right-hand side and denominator the left-hand side of the
conjectured inequality. We present numerical results that lead to some theorems and further conjectures, including variational analysis of the Crouzeix ratio at conjectured global minimizers. All the computations strongly support the truth of Crouzeix’s conjecture.

This is joint work with Anne Greenbaum and Adrian Lewis.
MSC Code: 
47A12