Variants of Khintchine's inequality with coefficients depending on the vector dimension are proved. Equality is attained for different types of extremal vectors. The Schur convexity of certain attached functions and direct estimates in terms of the Haagerup type of functions are also used.
Some characterizations of inner product spaces in terms of Birkhoff orthogonality are given. In this connection we define the rectangular modulus of the normed space . The values of the rectangular modulus at some noteworthy points are well-known constants of . Characterizations (involving of inner product spaces of dimension , respectively , are given and the behaviour of is studied.
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