The Nielsen–Thurston classification says that an orientation-preserving surface homeomorphism is periodic, reducible, or pseudo-Anosov. For a once-punctured torus, the three cases correspond to , , and for its action on first homology.
A periodic surface homeomorphism has a positive power equal to the identity. A mapping class is periodic when it has such a representative. Allowing motion of boundary circles is a different convention from requiring every isotopy to fix the boundary pointwise.
A pseudo-Anosov homeomorphism preserves two transverse measured foliations and stretches one while contracting the other by the same factor greater than one. Unlike the Anosov homeomorphism case, the foliations may have singularities.
An Anosov surface homeomorphism has nonsingular invariant expanding and contracting foliations. In the closed orientable surface setting it occurs on the torus, with a hyperbolic integral linear model.
The interior of the mapping torus of a pseudo-Anosov homeomorphism of a compact surface with boundary admits a complete finite-volume hyperbolic metric. Periodic monodromy instead gives a Seifert fibered mapping torus, while reducible monodromy gives a manifold containing an essential torus.
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The Nielsen–Thurston classification is a way of classifying the types of homeomorphisms on the surface of a two-dimensional manifold, particularly in the context of surfaces with hyperbolic geometry. It specifically deals with the study of homeomorphisms of compact surfaces, particularly orientable and non-orientable surfaces.