Two points $x$ and $y$ in a normed linear space $\mathbb{X}$ are said to be Birkhoff-James orthogonal (denoted by $x\perp_By$) if $|x+\lambda y|\geq|x|~~\text{for every scalar}~\lambda.$ James proved that in a normed linear space of dimension more than two, Birkhoff-James orthogonality is symmetric if and only if the parallelogram law holds. Motivated by this result, Sain introduced the concept of pointwise symmetry of Birkhoff-James orthogonality in a normed linear space.

In this talk, we shall try to understand the geometry of normed spaces in the light of Birkhoff-James orthogonality. After introducing the basic notations and terminologies, we begin with a study of the geometry of the normed algebra of holomorphic maps in a neighborhood of a curve and establish a relationship among the extreme points of the closed unit ball, Birkhoff-James orthogonality, and zeros of holomorphic maps.

We next study Birkhoff-James orthogonality and its pointwise symmetry in Lebesgue spaces defined on arbitrary measure spaces and natural numbers. We further find the onto isometries of the sequence spaces using the pointwise symmetry of the orthogonality.

We shall then study the geometry of tensor product spaces and use the results to study the relationship between the symmetry of orthogonality and the geometry (for example, extreme points and smooth points) of certain spaces of operators. Our work in this section is motivated by the famous Grothendieck inequality.

Finally, we study the geometry of $\ell_p$ and $c_0$ direct sums of normed spaces ($1\leq p<\infty$). We shall characterize the smoothness and approximate smoothness of these spaces along with Birkhoff-James orthogonality and its pointwise symmetry. As a consequence of our study we answer a question pertaining to the approximate smoothness of a space, raised by Chmeilienski, Khurana, and Sain.

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Last updated: 19 Apr 2024