Von Neumann's inequality

In operator theory, von Neumann's inequality, due to John von Neumann, states that, for a fixed contraction T, the polynomial functional calculus map is itself a contraction.

Statement

For a contraction T acting on a Hilbert space and a polynomial p, then the norm of p(T) is bounded by the supremum of |p(z)| for z in the unit disk."

Proof

The inequality can be proved by considering the unitary dilation of T, for which the inequality is obvious.

Generalizations

This inequality is a specific case of Matsaev's conjecture. That is that for any polynomial P and contraction T on $L^p$

$||P(T)|{|}_{L^p\to L^p}\le ||P(S)|{|}_{{\ell }^p\to {\ell }^p}$

where S is the right-shift operator. The von Neumann inequality proves it true for $p=2$ and for $p=1$ and $p=\mathrm{\infty }$ it is true by straightforward calculation. S.W. Drury has shown in 2011 that the conjecture fails in the general case.

See also

• Crouzeix's conjecture