ComputationalConformalGeometryLectureNotes
Topology, Differential Geometry, Complex Analysis
David GU
gu@cs.sunysb.edu
http://www.cs.sunysb.edu/˜gu
Computer Science Department
Stony Brook University
David Gu, Computer Science Department, Stony Brook University,
http://www.cs.sunysb.edu/˜gu – p. 1/97
DefinitionofManifold
A manifold of dimension n is a connected Hausdorfff space M for which every point has
a neighbourhood U that is homeomorphic to an open subset V of Rn. Such a
hemeomorphism
is called a coordinate chart. An atlas is a family of charts {Uα, φα} for which Uα
constitute an open covering of M.
φ : U → V
S
Uα
Uβ
φα
φβ
R2
φα(Uα)
φαβ
φβ (Uβ )
David Gu, Computer Science Department, Stony Brook University,
http://www.cs.sunysb.edu/˜gu – p. 2/97
Figure 1: Manifold. chart, atlas.
DifferentialManifold
• Transition function: Suppose {Uα, φα} and {Uβ , φβ} are two charts on a manifold
S, Uα ∩ Uβ 6= , the chart transition is
φαβ : φα(Uα ∩ Uβ ) → φβ (Uα ∩ Uβ )
• Differentiable Atlas: An atlas {Uα, φα} on a manifold is called differentiable if all
charts transitions are differentiable of class C∞.
• Differential Structure: A chart is called compatible with a differentiable atlas if
adding this chart to the atlas yields again a differentiable atlas. Taking all charts
compatible with a given differentiable atlas yieds a differentiable structure.
• differentiable manifold: A differentiable manifold of dimension n is a manifold of
dimension n together with a differentiable structure.
David Gu, Computer Science Department, Stony Brook University,
http://www.cs.sunysb.edu/˜gu – p. 3/97
DifferentialMap
• Differential Map:A continuous map h : M → M′ between differential manifolds M
and M′ with charts {Uα, φα} and {U′α, φ′α} is said to be differentiable if all the
maps φ′β ◦ hφ−1
α are differentiable of class C∞ wherever they are defined.
• Diffeomorphism: If h is a homeomorphism and if both h and h−1 are differentiable,
then h is called a diffeomorphism.
David Gu, Computer Science Department, Stony Brook University,
http://www.cs.sunysb.edu/˜gu – p. 4/97
RegularSurfacePatch
Suppose D = {(u, v)} is a planar domain, a map r : D → R3,
r(u, v) = (x(u, v), y(u, v), z(u, v)),
satisfying
1. x(u, v), y(u, v), z(u, v) are differentiable of class C∞.
2. ru and rv are linearly independent, namely
ru = (
∂x
∂u
,
∂y
∂u
,
∂z
∂u
), rv = (
∂x
∂v
,
∂y
∂v
,
∂z
∂v
),
∂z
∂u
), ru × rv 6= 0,
is a surface patch in R3, (u, v) are the coordinates parameters of the surface r.
David Gu, Computer Science Department, Stony Brook University,
http://www.cs.sunysb.edu/˜gu – p. 5/97
RegularSurfacePatch
n
ru
r(u0, v0)
rv
S
v
(u0, v0)
u
r(u, v)
Π
Figure 2: Surface patch.
David Gu, Computer Science Department, Stony Brook University,
http://www.cs.sunysb.edu/˜gu – p. 6/97
Differentparameterizations
Surface r can have different parameterizations. Consider a surface
r(u, v) : D → R3,
and parametric transformation
σ : (¯u, ¯v) ∈ ¯D → (u, v) ∈ D,
namely σ : ¯D → D is bijective and the J acobin
∂u(¯u,¯v)
∂u(¯u,¯v)
∂ ¯u
∂ ¯v
∂(u, v)
∂(¯u, ¯v)
=
∂v(¯u,¯v)
∂ ¯u
∂v(¯u,¯v)
∂ ¯v
6= 0.
then we have new parametric representation of the surface r,
r(¯u, ¯v) = r ◦ σ(¯u, ¯v) = r(u(¯u, ¯v), v(¯u, ¯v)) : ¯D → R3.
David Gu, Computer Science Department, Stony Brook University,
http://www.cs.sunysb.edu/˜gu – p. 7/97
Firstfundamentalform
Given a surface S in R3, r = r(u, v) is its parametric representation, denote
E =< ru, ru >, F =< ru, rv >, G =< rv, rv >,
the quadratic differential form
I = ds2 = Edu · du + 2F du · dv + Gdv · dv,
is called the first fundamental form of S.
David Gu, Computer Science Department, Stony Brook University,
http://www.cs.sunysb.edu/˜gu – p. 8/97