For any matrix A and vector x, the quadratic form x[sup]T[/sup]Ax is bounded above by the product of the maximum magnitude of any eigenvalue of A and the norm of x, and below by the product of the minimum magnitude of any eigenvalue of A and the norm of x. I can’t for the life of me remember what this is called.
Are you sure that inequality has a name? It’s not named here:Modelling and control of robot manipulators.
Is it a special case of the Wielandt inequality (which is supposedly equivalent to the Kantorovich inequality)?
Are you thinking of the Hausdorff-Toeplitz theorem?
It’s related to that, but not exactly the same. But that page did jog my memory: it’s the Rayleigh-Ritz inequality (and only holds for Hermitian matrices).