Sentence:  For it is clear that the total number of ordinary intersections of C and **f must be even ( otherwise , starting in the interior of C , **f could not finally return to the interior ) , and the center of rotation at t is the argument of the <WSD>function</WSD> , not a value . 
Meaning of the word marked with <WSD> tags:  (mathematics) a mathematical relation such that each element of a given set (the domain of the function) is associated with an element of another set (the range of the function)