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In mathematics, Milnor maps are named in honor of John Milnor, who introduced them to topology and algebraic geometry in his book Singular Points of Complex Hypersurfaces (Princeton University Press, 1968) and earlier lectures. The most studied Milnor maps are actually fibrations, and the phrase Milnor fibration is more commonly encountered in the mathematical literature. The general definition is as follows.
Let be a non-constant polynomial function of complex variables such that , so that the set of all complex -vectors with is a complex hypersurface of complex dimension containing the origin of complex -space. (For instance, if then is a complex plane curve containing .) The argument of is the function mapping the complement of in complex -space to the unit circle in C. For any real radius , the restriction of the argument of to the complement of in the real -sphere with center at the origin and radius is the Milnor map of at radius .
Milnor's Fibration Theorem states that, for every such that the origin is a singular point of the hypersurface (in particular, for every non-constant square-free polynomial of two variables, the case of plane curves), then for sufficiently small,
is a fibration. Each fiber is a non-compact differentiable manifold of real dimension . Note that the closure of each fiber is a compact manifold with boundary. Here the boundary corresponds to the intersection of with the -sphere (of sufficiently small radius) and therefore it is a real manifold of dimension . Furthermore, this compact manifold with boundary, which is known as the Milnor fiber (of the isolated singular point of at the origin), is diffeomorphic to the intersection of the closed -ball (bounded by the small -sphere) with the (non-singular) hypersurface where and is any sufficiently small non-zero complex number. This small piece of hypersurface is also called a Milnor fiber.
Milnor maps at other radii are not always fibrations, but they still have many interesting properties. For most (but not all) polynomials, the Milnor map at infinity (that is, at any sufficiently large radius) is again a fibration.