The cosine function is a classical tool for measuring angles in inner product spaces, and it has various extensions to normed linear spaces. In this paper, we investigate a cosine function for the convex angle formed by two nonzero elements of a complex normed linear space, in connection with recent results on the Birkhoff-James approximate orthogonality sets.

中文翻译：

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范线性空间的Birkhoff-James余弦函数

余弦函数是用于测量内积空间中角度的经典工具，并且对范数线性空间具有各种扩展。在本文中，我们结合Birkhoff-James近似正交集上的最新结果，研究了复范数线性空间中两个非零元素形成的凸角的余弦函数。