# 2-manifolds

A surface is a synonym for a 2-dimensional manifold. Also complex 2-dimensional (real 4-dimensional) complex manifolds are called surfaces. This article deals with real, compact, connected surfaces. Unless stated otherwise surfaces without boundary are considered.

## 1 First construction: connected sum

All orientable surfaces are homeomorphic to the connected sum of $g$$A surface is a synonym for a 2-dimensional manifold. Also complex 2-dimensional (real 4-dimensional) complex manifolds are called surfaces. This article deals with real, compact, connected surfaces. Unless stated otherwise surfaces without boundary are considered. == First construction: connected sum == ; All orientable surfaces are homeomorphic to the connected sum of g [[Torus|tori]] T^2 (g\geq 0). The case g=0 refers to the 2-[[sphere]] S^2. All non-orientable surfaces are homeomorphic to the connected sum of h [[real projective spaces]] \RP^2 (h\geq 1). For example, the ''Klein bottle'' is homeomorphic to \RP^2\mathbin{\sharp}\RP^2. The numbers g and h are called the ''genus'' of the orientable/non-orientable surface. [[Image:Surfaces.png|left|frame|A 2-sphere (genus 0), a torus (genus 1) and an orientable surface of higher genus]] [[Image:Boy surface.jpg|right|thumb|310px|The Boy surface, an immersion of \RP^2 in \Rr^3. This steel sculpture stands in front of the [http://www.mfo.de Oberwolfach Institute].]] [[Image:Klein_bottle.png|left|frame|A Klein bottle (non-orientable, genus 2) immersed in \Rr^3]]{{-|left}} The connected sum of \RP^2 with T^2 is homeomorphic to \RP^2\mathbin{\sharp}\RP^2\mathbin{\sharp}\RP^2. == Surfaces with boundary == The boundary of a surface is a disjoint union (possibly empty) of circles. Surfaces with boundary can be constructed as boundaryless surfaces with open 2-disks removed. == Classification == ; Compact, connected surfaces are classified by orientability (yes/no), the number of boundary components (a nonnegative integer) and the genus after filling the bounday circles by disks (an integer \geq 0 in the orientable case, \geq 1 in the non-orientable case). Instead of the genus, also e. g. the Euler characteristic can be used in the classification. The classifications up to homotopy equivalence, homeomorphism, PL-equivalence and diffeomorphism coincide. References: {{cite|Ahlfors&Sario1960|Thm. 1.42A, 1§8}}, {{cite|Hirsch1994|Thm. 9.3.11}}, {{cite|Moise1977|Thm. 8.5, Thm. 22.9}}{{-}} == More constructions == === By polygons === ; Each orientable surface of genus g>0 can be constructed by identifying pairs of edges in a regular polygon with g sides. Label the edges by the sequence a_1,b_1,\bar a_1,\bar b_1,\ldots,a_g,b_g,\bar a_g,\bar b_g. Also orient the edges such that those labeled without an overbar are oriented in one direction (e. g. clockwise) and those with an overbar are oppositely oriented. Now identify corresponding edges, respecting the orientation. The 2-sphere can be obtained from a 2-gon with edges labeled a,\bar a. Each non-orientable surface of genus h can be obtained from a h-gon with edges labeled a_1,a_1,\ldots,a_h,a_h. {| | [[Image:Polygon_construction.png|frame|The orientable surface of genus 2 can be obtained by identifying edges in a regular octagon.]] | [[Image:Polygon_sphere.png|frame|The 2-sphere can be obtained by identifying the edges of a 2-gon.]] | [[Image:Polygon_RP2.png|frame|Construction of \RP^2.]] | [[Image:Polygon_Klein_bottle.png|frame|Construction of the Klein bottle.]] |} Reference: {{cite|Massey1977|Section 1.5}} === By gluing handles and crosscaps === ; An orientable surface of genus g can be obtained by successively gluing g 1-handles to the 2-sphere such that the embeddings of S^0\times D^2 in S_{g-1} are in each case either orientation-preserving or orientation-reversing on both components \{1\}\times D^2\cong D^2 and \{-1\}\times D^2\cong -D^2. An non-orientable surface of genus h can be obtained by gluing h ''crosscaps'' to S^2. For this, embed D^2 in S^2 (or R_{h-1} from the second crosscap on), remove the interior and glue in the Möbius strip, which also has boundary S^1. The result of attaching a non-orientable handle to S^2 or any handle to a non-orientable surface is diffeomorphic to the surface with two additional crosscaps. Reference: {{cite|Hirsch1994|Section 9.1}} === By branched coverings === ; Every orientable surface of genus g can be obtained as the branched double covering of the 2-sphere with g+2 branching points. === As complex curves === ; A smooth, irreducible, plane, complex projective-algebraic curve of order d (i. e. the zero set of a non-constant, homogeneous, irreducible polynomial of degree d in \CP^2 whose gradient vector does not vanish in any point of the zero set) is a compact, connected, orientable, real surface of genus g = \frac{(d-1)(d-2)}2. Furthermore, surfaces of every degree can be obtained as normalizations of plane, complex projective-algebraic curves with only double points as singularities. Reference: {{cite|Brieskorn&Knörrer1986|Lemma 9.2.1}} == Properties == === Orientable surfaces === ; Let S_g denote an oriented surface of genus g. * By the polygon construction above, each orientable surface has a cell decomposition with one 0-cell, g 1-cells and one 2-cell. All differentials in the chain complex are zero maps. * Therefore, the homology groups with any coefficients G are given by H_0\cong G, H_1\cong G^{2g}, H_2\cong G. * The integral cohomology ring is completely determined by the intersection form on H^1, which is necessarily isomorphic to the hyperbolic form. A basis for H^1(S_g), for which the intersection form is the standard hyperbolic form, is given by the loops (a_1,b_1,\ldots,a_g,b_g) in the polygon construction above. * Homotopy groups ** For the homotopy group of S^2, see the article about [[sphere|spheres]]. ** All surfaces of higher genus g\geq1 are aspherical with fundamental group \pi_1(S_g,*)\cong \langle a_1,b_1,\ldots, a_g,b_g \mid a_1b_1\cdots a_gb_ga_1^{-1}b_1^{-1}\cdots a_g^{-1}b_g^{-1}\rangle. In particular, the fundamental group of the 2-torus is isomorphic to the abelian group \Zz^2. * All orientable surfaces can be embedded in \Rr^3. * Every surface can be given a complex structure. Together with the complex structure, it is a [[wikipedia:Riemann surface|Riemann surface]] * Characteristic classes ** All Stiefel-Whitney classes vanish. ** All Pontrjagin classes vanish. ** The Euler characteristic is -2g. ** Given a complex structure on the surface, the first Chern class is equal to the Euler class. * All orientable surfaces admit metrics with constant curvature: the standard metric on the unit sphere in \Rr^3 has constant positive curvature, the torus can be given a flat metric, and all surfaces of higher genus admit metrics of constant negative curvature. * All surfaces are smoothly [[amphicheiral]]. === Non-orientable surfaces === ; Let R_h denote an non-orientable surface of genus h. * Again, each non-orientable surface has a cell decomposition with one 0-cell, h 1-cells and one 2-cell. The differential C_1\to C_0 is the zero map, while the differential C_2\to C_1 with respect to the basis given by the loops the loops (a_1,\ldots,a_h) in the polygon construction is the matrix (2,\ldots, 2). * Therefore, the integral homology groups are isomorphic to H_0\cong\Zz, H_1\cong \Zz^{h-1}\oplus \Zz/2, H_2\cong 0. * The mod-2 homology groups are H_0=\Zz/2, H_1\cong(\Zz/2)^h, H_2=\Zz/2 since the chain complex above is acyclic mod 2. The intersection form on H_1(R_h;\Zz/2) with respect to this cellular basis is given by the identity matrix. * Homotopy groups ** The fundamental group of R_h is \pi_1(R_h,*)\cong \langle a_1,\ldots, a_h \mid a_1^2\cdots a_h^2\rangle. ** The orientation double covering of R_h is S_{h-1}. Therefore, all higher homotopy groups of \RP^2 equal those of S^2, and the non-orientable surfaces of higher genus are aspherical. * The orientation double covering also determines the curvature properties: \RP^2 can be given a metric with constant positive curvature, the Klein bottle a flat metric, and all nonorientable surfaces can be given metrics with constant negative curvature. * Non-orientable surfaces cannot be embedded in \Rr^3. * Characteristic classes ** The first Stiefel-Whitney class is the orientation character. It can be described by the homomorphism \pi_1(R_h,*)\to\Zz/2 which maps each generator a_1,\ldots,a_h to the generator of \Zz/2. The second Stiefel-Whitney class is zero if h is even and the non-zero element of H^2(R_h;\Zz)=\Zz/2 if h is odd. ** The Euler characteristic is -h. == References == {{#RefList:}} [[Category:Manifolds]] [[Category:Aspherical manifolds]] [[Category:Constant curvature]]g$ tori $T^2$$T^2$ ($g\geq 0$$g\geq 0$). The case $g=0$$g=0$ refers to the 2-sphere $S^2$$S^2$. All non-orientable surfaces are homeomorphic to the connected sum of $h$$h$ real projective spaces $\RP^2$$\RP^2$ ($h\geq 1$$h\geq 1$). For example, the Klein bottle is homeomorphic to $\RP^2\mathbin{\sharp}\RP^2$$\RP^2\mathbin{\sharp}\RP^2$. The numbers $g$$g$ and $h$$h$ are called the genus of the orientable/non-orientable surface.

A 2-sphere (genus 0), a torus (genus 1) and an orientable surface of higher genus
The Boy surface, an immersion of $\RP^2$$\RP^2$ in $\Rr^3$$\Rr^3$. This steel sculpture stands in front of the Oberwolfach Institute.
A Klein bottle (non-orientable, genus 2) immersed in $\Rr^3$$\Rr^3$

The connected sum of $\RP^2$$\RP^2$ with $T^2$$T^2$ is homeomorphic to $\RP^2\mathbin{\sharp}\RP^2\mathbin{\sharp}\RP^2$$\RP^2\mathbin{\sharp}\RP^2\mathbin{\sharp}\RP^2$.

## 2 Surfaces with boundary

The boundary of a surface is a disjoint union (possibly empty) of circles. Surfaces with boundary can be constructed as boundaryless surfaces with open 2-disks removed.

## 3 Classification

Compact, connected surfaces are classified by orientability (yes/no), the number of boundary components (a nonnegative integer) and the genus after filling the bounday circles by disks (an integer $\geq 0$$\geq 0$ in the orientable case, $\geq 1$$\geq 1$ in the non-orientable case). Instead of the genus, also e. g. the Euler characteristic can be used in the classification. The classifications up to homotopy equivalence, homeomorphism, PL-equivalence and diffeomorphism coincide.

References: [Ahlfors&Sario1960, Thm. 1.42A, 1§8], [Hirsch1994, Thm. 9.3.11], [Moise1977, Thm. 8.5, Thm. 22.9]

## 4 More constructions

### 4.1 By polygons

Each orientable surface of genus $g>0$$g>0$ can be constructed by identifying pairs of edges in a regular polygon with $4g$$4g$ sides. Label the edges by the sequence $a_1,b_1,\bar a_1,\bar b_1,\ldots,a_g,b_g,\bar a_g,\bar b_g$$a_1,b_1,\bar a_1,\bar b_1,\ldots,a_g,b_g,\bar a_g,\bar b_g$. Also orient the edges such that those labeled without an overbar are oriented in one direction (e. g. clockwise) and those with an overbar are oppositely oriented. Now identify corresponding edges, respecting the orientation.

The 2-sphere can be obtained from a 2-gon with edges labeled $a,\bar a$$a,\bar a$.

Each non-orientable surface of genus $h$$h$ can be obtained from a $2h$$2h$-gon with edges labeled $a_1,a_1,\ldots,a_h,a_h$$a_1,a_1,\ldots,a_h,a_h$.

 The orientable surface of genus 2 can be obtained by identifying edges in a regular octagon. The 2-sphere can be obtained by identifying the edges of a 2-gon. Construction of $\RP^2$$\RP^2$. Construction of the Klein bottle.

Reference: [Massey1977, Section 1.5]

### 4.2 By gluing handles and crosscaps

An orientable surface of genus $g$$g$ can be obtained by successively gluing $g$$g$ 1-handles to the 2-sphere such that the embeddings of $S^0\times D^2$$S^0\times D^2$ in $S_{g-1}$$S_{g-1}$ are in each case either orientation-preserving or orientation-reversing on both components $\{1\}\times D^2\cong D^2$$\{1\}\times D^2\cong D^2$ and $\{-1\}\times D^2\cong -D^2$$\{-1\}\times D^2\cong -D^2$.

An non-orientable surface of genus $h$$h$ can be obtained by gluing $h$$h$ crosscaps to $S^2$$S^2$. For this, embed $D^2$$D^2$ in $S^2$$S^2$ (or $R_{h-1}$$R_{h-1}$ from the second crosscap on), remove the interior and glue in the Möbius strip, which also has boundary $S^1$$S^1$. The result of attaching a non-orientable handle to $S^2$$S^2$ or any handle to a non-orientable surface is diffeomorphic to the surface with two additional crosscaps.

Reference: [Hirsch1994, Section 9.1]

### 4.3 By branched coverings

Every orientable surface of genus $g$$g$ can be obtained as the branched double covering of the 2-sphere with $2g+2$$2g+2$ branching points.

### 4.4 As complex curves

A smooth, irreducible, plane, complex projective-algebraic curve of order $d$$d$ (i. e. the zero set of a non-constant, homogeneous, irreducible polynomial of degree $d$$d$ in $\CP^2$$\CP^2$ whose gradient vector does not vanish in any point of the zero set) is a compact, connected, orientable, real surface of genus

$\displaystyle g = \frac{(d-1)(d-2)}2.$

Furthermore, surfaces of every degree can be obtained as normalizations of plane, complex projective-algebraic curves with only double points as singularities.

Reference: [Brieskorn&Knörrer1986, Lemma 9.2.1]

## 5 Properties

### 5.1 Orientable surfaces

Let $S_g$$S_g$ denote an oriented surface of genus $g$$g$.

• By the polygon construction above, each orientable surface has a cell decomposition with one 0-cell, $2g$$2g$ 1-cells and one 2-cell. All differentials in the chain complex are zero maps.
• Therefore, the homology groups with any coefficients $G$$G$ are given by $H_0\cong G$$H_0\cong G$, $H_1\cong G^{2g}$$H_1\cong G^{2g}$, $H_2\cong G$$H_2\cong G$.
• The integral cohomology ring is completely determined by the intersection form on $H^1$$H^1$, which is necessarily isomorphic to the hyperbolic form. A basis for $H^1(S_g)$$H^1(S_g)$, for which the intersection form is the standard hyperbolic form, is given by the loops $(a_1,b_1,\ldots,a_g,b_g)$$(a_1,b_1,\ldots,a_g,b_g)$ in the polygon construction above.
• Homotopy groups
• For the homotopy group of $S^2$$S^2$, see the article about spheres.
• All surfaces of higher genus $g\geq1$$g\geq1$ are aspherical with fundamental group $\pi_1(S_g,*)\cong \langle a_1,b_1,\ldots, a_g,b_g \mid a_1b_1\cdots a_gb_ga_1^{-1}b_1^{-1}\cdots a_g^{-1}b_g^{-1}\rangle$$\pi_1(S_g,*)\cong \langle a_1,b_1,\ldots, a_g,b_g \mid a_1b_1\cdots a_gb_ga_1^{-1}b_1^{-1}\cdots a_g^{-1}b_g^{-1}\rangle$. In particular, the fundamental group of the 2-torus is isomorphic to the abelian group $\Zz^2$$\Zz^2$.
• All orientable surfaces can be embedded in $\Rr^3$$\Rr^3$.
• Every surface can be given a complex structure. Together with the complex structure, it is a Riemann surface
• Characteristic classes
• All Stiefel-Whitney classes vanish.
• All Pontrjagin classes vanish.
• The Euler characteristic is $2-2g$$2-2g$.
• Given a complex structure on the surface, the first Chern class is equal to the Euler class.
• All orientable surfaces admit metrics with constant curvature: the standard metric on the unit sphere in $\Rr^3$$\Rr^3$ has constant positive curvature, the torus can be given a flat metric, and all surfaces of higher genus admit metrics of constant negative curvature.

### 5.2 Non-orientable surfaces

Let $R_h$$R_h$ denote an non-orientable surface of genus $h$$h$.

• Again, each non-orientable surface has a cell decomposition with one 0-cell, $h$$h$ 1-cells and one 2-cell. The differential $C_1\to C_0$$C_1\to C_0$ is the zero map, while the differential $C_2\to C_1$$C_2\to C_1$ with respect to the basis given by the loops the loops $(a_1,\ldots,a_h)$$(a_1,\ldots,a_h)$ in the polygon construction is the matrix $(2,\ldots, 2)$$(2,\ldots, 2)$.
• Therefore, the integral homology groups are isomorphic to $H_0\cong\Zz$$H_0\cong\Zz$, $H_1\cong \Zz^{h-1}\oplus \Zz/2$$H_1\cong \Zz^{h-1}\oplus \Zz/2$, $H_2\cong 0$$H_2\cong 0$.
• The mod-2 homology groups are $H_0=\Zz/2$$H_0=\Zz/2$, $H_1\cong(\Zz/2)^h$$H_1\cong(\Zz/2)^h$, $H_2=\Zz/2$$H_2=\Zz/2$ since the chain complex above is acyclic mod 2. The intersection form on $H_1(R_h;\Zz/2)$$H_1(R_h;\Zz/2)$ with respect to this cellular basis is given by the identity matrix.
• Homotopy groups
• The fundamental group of $R_h$$R_h$ is $\pi_1(R_h,*)\cong \langle a_1,\ldots, a_h \mid a_1^2\cdots a_h^2\rangle$$\pi_1(R_h,*)\cong \langle a_1,\ldots, a_h \mid a_1^2\cdots a_h^2\rangle$.
• The orientation double covering of $R_h$$R_h$ is $S_{h-1}$$S_{h-1}$. Therefore, all higher homotopy groups of $\RP^2$$\RP^2$ equal those of $S^2$$S^2$, and the non-orientable surfaces of higher genus are aspherical.
• The orientation double covering also determines the curvature properties: $\RP^2$$\RP^2$ can be given a metric with constant positive curvature, the Klein bottle a flat metric, and all nonorientable surfaces can be given metrics with constant negative curvature.
• Non-orientable surfaces cannot be embedded in $\Rr^3$$\Rr^3$.
• Characteristic classes
• The first Stiefel-Whitney class is the orientation character. It can be described by the homomorphism $\pi_1(R_h,*)\to\Zz/2$$\pi_1(R_h,*)\to\Zz/2$ which maps each generator $a_1,\ldots,a_h$$a_1,\ldots,a_h$ to the generator of $\Zz/2$$\Zz/2$. The second Stiefel-Whitney class is zero if $h$$h$ is even and the non-zero element of $H^2(R_h;\Zz)=\Zz/2$$H^2(R_h;\Zz)=\Zz/2$ if $h$$h$ is odd.
• The Euler characteristic is $2-h$$2-h$.