Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In mathematics, a set S in the Euclidean space Rn is called a star domain (or star-convex set) if there exists x0 in S such that for all x in S the line segment from x0 to x is in S. This definition is immediately generalizable to any real or complex vector space. Intuitively, if one thinks of S as of a region surrounded by a fence, S is a star domain if one can find a vantage point x0 in S from which any point x in S is within line-of-sight. Mathematics is the study of quantity, structure, space, and change. Mathematicians seek out patterns, formulate new conjectures, and establish truth by rigorous deduction from appropriately chosen axioms and definitions.