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Schematic figures of a limit cycle and isophase hypersurfaces. (a) An isochron Iϕ is a n − 1-dimensional hypersurface, which intersects with the limit cycle C at the point χ(ϕ). (b) An isophase Eϕ used in Method II is identified with the tangent space of Iϕ at point χ(ϕ). (Online version in colour.)
