Moving object and observer in a Banach space

For a moving object and an observer in a smooth strictly convex Banach space, a function characterizing the visibility of the object to the observer is introduced. The position of the observer is given inaccurately. The differentiability property of this function is investigated; in the case of a finite-dimensional space, an expression is found for the derivative in any direction. The visibility characteristic for a group of observers is studied.