We define pr(s) as pr(s) = 1 α k , where k is the distance (counted in terms of the number of sentences) between sentence s and the center sentence of the window containing s; by "center sentence", we mean the citing sentence containing the citation label.
the sentence with the citation label will have a proximity of 1 (because k = 0), while the sentences away from the citation label will have a decaying weight controlled by parameter α.