Abstract
A point x in real Hilbert space is represented by an infinite sequence (x1, x2, x3, …) of real numbers such thatis convergent. The unit “sphere“ S consists of all points × for which ‖x‖ ≤ 1. The sphere of radius a and centre y is denoted by Sa(y) and consists of all points × for which ‖x−y‖ ≤ a.

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