Genus g surface#Genus 2
{{DISPLAYTITLE:Genus g surface}}
{{short description|Smooth closed surface with g holes}}
In mathematics, a genus g surface (also known as a g-torus or g-holed torus) is a surface formed by the connected sum of g distinct tori: the interior of a disk is removed from each of g distinct tori and the boundaries of the g many disks are identified (glued together), forming a g-torus. The genus of such a surface is g.
A genus g surface is a two-dimensional manifold. The classification theorem for surfaces states that every compact connected two-dimensional manifold is homeomorphic to either the sphere, the connected sum of tori, or the connected sum of real projective planes.
Definition of genus
{{Main|Genus (mathematics)}}
The genus of a connected orientable surface is an integer representing the maximum number of cuttings along non-intersecting closed simple curves without rendering the resultant manifold disconnected.Munkres, James R. Topology. Vol. 2. Upper Saddle River: Prentice Hall, 2000. It is equal to the number of handles on it. Alternatively, it can be defined in terms of the Euler characteristic χ, via the relationship χ = 2 − 2g for closed surfaces, where g is the genus.
The genus (sometimes called the demigenus or Euler genus) of a connected non-orientable closed surface is a positive integer representing the number of cross-caps attached to a sphere. Alternatively, it can be defined for a closed surface in terms of the Euler characteristic χ, via the relationship χ = 2 − g, where g is the non-orientable genus.
Genus 0
An orientable surface of genus zero is the sphere S2. Another surface of genus zero is the disc.
File:Sphere filled blue.svg|A sphere
File:1-ball.svg|A closed disc (with boundary)
File:Minimal_4_colour_sphere.svg|By the Heawood conjecture, it can be coloured with up to 4 mutually adjacent regions
Genus 1
A genus one orientable surface is the ordinary torus. A non-orientable surface of genus one is the projective plane.{{cite book| first=Glen E. | last=Bredon | author-link=Glen Bredon | title=Topology and Geometry | publisher=Springer-Verlag | year=1993 | isbn=0-387-97926-3}}
Elliptic curves over the complex numbers can be identified with genus 1 surfaces. The formulation of elliptic curves as the embedding of a torus in the complex projective plane follows naturally from a property of Weierstrass's elliptic functions that allows elliptic curves to be obtained from the quotient of the complex plane by a lattice.{{Cite book
| last = Silverman | first=Joseph H. | author-link=Joseph H. Silverman
| year = 1986
| title = The Arithmetic of Elliptic Curves
| series = Graduate Texts in Mathematics
| volume=106
| publisher = Springer-Verlag
| isbn=0-387-96203-4
}}
File:Torus illustration.png|A torus of genus 1
File:7_colour_torus.svg|It can be coloured with up to 7 mutually adjacent regions
File:Elliptic curve simple.svg|An elliptic curve
Genus 2
The term double torus is occasionally used to denote a genus 2 surface.{{MathWorld |title=Double Torus |urlname=DoubleTorus}}{{cite journal |last1=Mayorga |first1=Luis S. |last2=Masone |first2=Diego |title=The Secret Ballet Inside Multivesicular Bodies |journal=ACS Nano |date=2024 |volume=18 |issue=24 |pages=15651–15660 |doi=10.1021/acsnano.4c01590|pmid=38830824 }}
A non-orientable surface of genus two is the Klein bottle.
The Bolza surface is the most symmetric Riemann surface of genus 2, in the sense that it has the largest possible conformal automorphism group.{{citation|last=Bolza|first= Oskar|title=On Binary Sextics with Linear Transformations into Themselves|journal= American Journal of Mathematics|volume= 10|issue= 1|year= 1887|pages= 47–70|jstor=2369402|doi=10.2307/2369402}}
File:Double torus illustration.png|A torus of genus 2
File:Tietze_genus_2_colouring.svg|It can be coloured with up to 8 mutually adjacent regions
Genus 3
{{for|the three-dimensional space|3-torus}}
The term triple torus is also occasionally used to denote a genus 3 surface.{{MathWorld |title=Triple Torus |urlname=TripleTorus}}
The Klein quartic is a compact Riemann surface of genus {{math|3}} with the highest possible order automorphism group for compact Riemann surfaces of genus 3. It has {{math|168}} orientation-preserving automorphisms, and {{math|336}} automorphisms altogether.
File:Sphere with three handles.png|A sphere with three handles
File:Triple torus array.png|The connected sum of three tori
File:Triple torus illustration.png|Triple torus
File:Taxel_genus_3_colouring.svg|It can be coloured with up to 9 mutually adjacent regions
File:Dodecagon with opposite faces identified.svg|Dodecagon with opposite edges identifiedJürgen Jost, (1997) "Compact Riemann Surfaces: An Introduction to Contemporary Mathematics", Springer
File:14-gon with opposite faces identified.svg|Tetradecagon with opposite edges identified
See also
References
{{reflist}}
Sources
- James R. Munkres, Topology, Second Edition, Prentice-Hall, 2000, {{ISBN|0-13-181629-2}}.
- William S. Massey, Algebraic Topology: An Introduction, Harbrace, 1967.