Surface
From Free net encyclopedia
- For other senses of this word, see surface (disambiguation).
Image:Saddle pt.jpg In mathematics (topology), a surface is a two-dimensional manifold. Examples arise in three-dimensional space as the boundaries of three-dimensional solid objects. The surface of a fluid object, such as a rain drop or soap bubble, is an idealisation. To speak of the surface of a snowflake, which has a great deal of fine structure, is to go beyond the simple mathematical definition. For the nature of real surfaces see surface tension, surface chemistry, surface energy, roughness.
The two-dimensional character of a surface comes from the fact that, about each point, there is a "coordinate patch" on which a two-dimensional coordinate system is defined; in general, it is not possible to extend this patch to the entire surface, so it will be necessary to define multiple patches which collectively cover the surface.
A surface may have a boundary, where the surface ends. For example, the boundary of a disc or hemisphere would be the circle around the edge.
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Examples
The general concept of a surface, and the richness and variety of surfaces, can be understood by examining a variety of examples. Any formal definition of a surface must be strong enough to encompass this variety.
- Parametric surfaces are surfaces defined by parametric equations, generally the images of maps from a sub set of R2 to R3.
- Developable surfaces are surfaces that can be flattened to a plane without stretching; examples include the cylinder, the cone, and the torus.
- Ruled surfaces are surfaces that have at least one straight line running through every point. Examples include the cylinder and the hyperboloid of one sheet.
- Surfaces of revolution are surfaces with cylindrical symmetry.
- Minimal surfaces are surfaces that minimize the surface area. Examples include soap films stretched across a wire frame, catenoids and helicoids.
- Algebraic surfaces are surfaces defined by the locus of zeros of a set of algebraic equations. Examples include the quadrics, the cubic surfaces, and the Veronese surface.
- Implicit surfaces are surfaces defined as the locus of zeros of a general equation.
- The Klein bottle and the Möbius strip are examples of non-orientable manifolds.
- Riemann surfaces are surfaces that admit a complex analytic structure; in particular, complex analytic functions may be defined between them. Examples include the sphere and the torus.
- Projective surfaces are defined in projective spaces. Examples include Boy's surface, the Roman surface and the cross-cap, all of which are Steiner surfaces.
- The Alexander horned sphere is an example of a surface with a topology that combines an ordinary smooth surface with a Cantor set.
Definition
In what follows, all surfaces are considered to be second-countable 2-dimensional manifolds.
More precisely: a topological surface (with boundary) is a Hausdorff space in which every point has an open neighbourhood homeomorphic to either an open subset of E2 (Euclidean 2-space) or an open subset of the closed half of E2. The set of points which have an open neighbourhood homeomorphic to En is called the interior of the manifold; it is always non-empty. The complement of the interior, is called the boundary; it is a (1)-manifold, or union of closed curves.
A surface with empty boundary is said to be closed if it is compact, and open if it is not compact.
Classification of closed surfaces
There is a complete classification of closed (i.e compact without boundary) connected, surfaces up to homeomorphism. Any such surface falls into one of two infinite collections:
- Spheres with g handles attached (called g-fold tori). These are orientable surfaces with Euler characteristic 2-2g, also called surfaces of genus g.
- Spheres with k projective planes attached. These are non-orientable surfaces with Euler characteristic 2-k.
Therefore Euler characteristic and orientability describe a compact surfaces up to homeomorphism (and if surfaces are smooth then up to diffeomorphism).
Compact surfaces
Compact surfaces with boundary are just these with one or more removed open disks whose closures are disjoint.
Embeddings in R3
A compact surface can be embedded in R3 if it is orientable or if it has nonempty boundary. It is a consequence of the Whitney embedding theorem that any surface can be embedded in R4.
Differential geometry
A simple review of the embedding of a surface in n dimensions, and a computation of the area of such a surface, is provided in the article volume form. Metric properties of Riemann surfaces are briefly reviewed in the article Poincaré metric.
Some models
To make some models of various surfaces, attach the sides of these squares (A with A, B with B) so that the directions of the arrows match:
Fundamental polygon
Each closed surface can be constructed from an even sided oriented polygon, called a fundamental polygon by pairwise identification of its edges.
This construction can be represented as a string of length 2n of n distinct symbols where each symbol appears twice with exponent either +1 or -1. The exponent -1 signifies that the corresponding edge has the orientation opposing the one of the fundamental polygon.
The above models can be described as follows:
- sphere: <math>A A^{-1}</math>
- projective plane: <math>A A</math>
- Klein bottle: <math>A B A^{-1} B</math>
- torus: <math>A B A^{-1} B^{-1}</math>
(See the main article fundamental polygon for details.)
Connected sum of surfaces
Given two surfaces M and M', their connected sum M # M' is obtained by removing a disk in each of them and gluing them along the newly formed boundary components.
We use the following notation.
- sphere: S
- torus: T
- Klein bottle: K
- Projective plane: P
Facts:
- S # S = S
- S # M = M
- P # P = K
- P # K = P # T
We use a shorthand natation: nM = M # M # ... # M (n-times) with 0M = S.
Closed surfaces are classified as follows:
- gT (g-fold torus): orientable surface of genus g, for <math>g \ge 0</math>.
- gP (g-fold projective plane): non-orientable surface of genus g, for <math>g \ge 1</math>.
Algebraic surface
This notion of a surface is distinct from the notion of an algebraic surface. A non-singular complex projective algebraic curve is a smooth surface. Algebraic surfaces over the complex number field have dimension 4 when considered as a real manifold. Algebraic surfaces over the real numbers will give normal surfaces, however these may contain singular points, where the algebraic surface forms a degenerate lines or points.
External links
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