Further results on $\mathbb{A}$-numerical radius inequalities

Abstract: Let $A$ be a bounded linear positive operator on a complex Hilbert space $\mathcal{H}.$ Further, let $\mathcal{B}_A\mathcal{(H)}$ denote the set of all bounded linear operators on $\mathcal{H}$ whose $A$-adjoint exists, and $\mathbb{A}$ signify a diagonal operator matrix with diagonal entries are $A.$ Very recently, several $A$-numerical radius inequalities of $2\times 2 $ operator matrices were established by Feki and Sahoo [arXiv:2006.09312; 2020] and Bhunia {\it et al.} [Linear Multilinear Algebra (2020), DOI: 10.1080/03081087.2020.1781037], assuming the conditions "$\mathcal{N}(A)^\perp$ is invariant under different operators in $\mathcal{B}_A(\mathcal{H})$" and "$A$ is strictly positive", respectively. In this paper, we prove a few new $\mathbb{A}$-numerical radius inequalities for $2\times 2$ and $n\times n$ operator matrices. We also provide some new proofs of the existing results by relaxing different sufficient conditions like "$\mathcal{N}(A)^\perp$ is invariant under different operators" and "$A$ is strictly positive". Our proofs show the importance of the theory of the Moore-Penrose inverse of a bounded linear operator in this field of study.