Noncommutative Buzano-Dragomir Inequality and Applications

Dragomir [Bull. Aust. Math. Soc., 2016] showed that the Buzano inequality holds for orthogonal projections on Hilbert spaces. Dragomir [Linear Multilinear Algebra, 2016] also derived the most general form of the Buzano inequality for bounded linear operators on Hilbert spaces. We show that Dragomir result extends to adjointable morphisms on Hilbert C*-modules. Using this generalization, we derive bounds for the roots of polynomials over commutative unital C*-algebras. We formulate the notion of noncommutative numerical range and derive numerical radius bounds for self-adjoint morphisms on Hilbert C*-modules over unital C*-algebras. We formulate several open problems, including the noncommutative Toeplitz-Hausdorff, von Neumann inequality, Ando inequality, Berger power dilation, Kittaneh inequality, Crouzeix problems.