On rectangular constant in normed linear spaces

Abstract

We study the properties of rectangular constant $ μ(\mathbb{X}) $ in a normed linear space $\mathbb{X}$. We prove that $ μ(\mathbb{X}) = 3$ iff the unit sphere contains a straight line segment of length 2. In fact, we prove that the rectangular modulus attains its upper bound iff the unit sphere contains a straight line segment of length 2. We prove that if the dimension of the space $\mathbb{X}$ is finite then $μ(\mathbb{X})$ is attained. We also prove that a normed linear space is an inner product space iff we have sup$\{\frac{1+|t|}{\|y+tx\|}$: $x,y \in S_{\mathbb{X}}$ with $x\bot_By\} \leq \sqrt{2}$ $\forall t$ satisfying $|t|\in (3-2\sqrt{2},\sqrt{2}+1)$.

Theorems & Definitions (13)

• Lemma 2.1
• Theorem 2.2
• Remark 2.3
• Theorem 3.1
• Theorem 3.2
• Theorem 3.3
• Corollary 3.4
• Corollary 3.5
• Theorem 3.6
• Lemma 3.7