词条 | Rotation number |
释义 |
This article is about the rotation number, which is sometimes called the map winding number or simply winding number. There is another meaning for winding number, which appears in complex analysis. In mathematics, the rotation number is an invariant of homeomorphisms of the circle. HistoryIt was first defined by Henri Poincaré in 1885, in relation to the precession of the perihelion of a planetary orbit. Poincaré later proved a theorem characterizing the existence of periodic orbits in terms of rationality of the rotation number. DefinitionSuppose that f: S1 → S1 is an orientation preserving homeomorphism of the circle S1 = R/Z. Then f may be lifted to a homeomorphism F: R → R of the real line, satisfying for every real number x and every integer m. The rotation number of f is defined in terms of the iterates of F: Henri Poincaré proved that the limit exists and is independent of the choice of the starting point x. The lift F is unique modulo integers, therefore the rotation number is a well-defined element of R/Z. Intuitively, it measures the average rotation angle along the orbits of f. ExampleIf f is a rotation by 2πθ (where 0≤θ<2π), then then its rotation number is θ (cf Irrational rotation). PropertiesThe rotation number is invariant under topological conjugacy, and even topological semiconjugacy: if f and g are two homeomorphisms of the circle and for a continuous map h of the circle into itself (not necessarily homeomorphic) then f and g have the same rotation numbers. It was used by Poincaré and Arnaud Denjoy for topological classification of homeomorphisms of the circle. There are two distinct possibilities.
The rotation number is continuous when viewed as a map from the group of homeomorphisms (with topology) of the circle into the circle. See also
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2 : Fixed points (mathematics)|Dynamical systems |
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