Sentence:  In the neighborhood of an end point of an interval of tangent points in the f-plane the function is two valued or no valued on one side , and is a single valued <WSD>function</WSD> consisting entirely of tangent points on the other side . 
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)