# Parameterized Surfaces

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1 Parameterized Surfaces Definition: A parameterized surface x : U R 2 R 3 is a differentiable map x from an open set U R 2 into R 3. The set x(u) R 3 is called the trace of x. x is regular if the differential dx q : R 2 R 3 is one-to-one for all q U (i.e., the vectors x/ u, x/ v are linearly independent for all q U). A point p U where dx p is not one-toone is called a singular point of x.

2 Proposition: Let x : U R 2 R 3 be a regular parameterized surface and let q U. Then there exists a neighborhood V of q in R 2 such that x(v ) R 3 is a regular surface.

3 Tangent Plane Definition 1: By a tangent vector to a regular surface S at a point p S, we mean the tangent vector α (0) of a differentiable parameterized curve α : ( ɛ, ɛ) S with α(0) = p. Proposition 1: Let x : U R 2 S be a parameterization of a regular surface S and let q U. The vector subspace of dimension 2, dx q (R 2 ) R 3 coincides with the set of tangent vectors to S at x(q).

4 Definition 2: By Proposition 1, the plane dx q (R 2 ), which passes through x(q) = p, does not depend on the parameterization x. This plane is called the tangent plane to S at p and will be denoted by T p (S). The choice of the parameterization x determines a basis {( x/ u)(q), ( x/ v)(q)} of T p (S), called the basis associated to x.

5 The coordinates of a vector w T p (S) in the basis associated to a parameterization x are determined as follows: w is the velocity vector α (0) of a curve α = x β, where β : ( ɛ, ɛ) U is given by β(t) = (u(t), v(t)), with β(0) = q = x 1 (p). Thus, α (0) = d dt (x β)(0) = d x(u(t), v(t))(0) dt = x u (q)u (0) + x v (q)v (0) = w Thus, in the basis {x u (q), x v (q)}, w has coordinates (u (0), v (0)), where (u(t), v(t)) is the expression of a curve whose velocity vector at t = 0 is w.

6 Let S 1 and S 2 be two regular surfaces and let ϕ : V S 1 S 2 be a differentiable mapping of an open set V of S 1 into S 2. If p V, then every tangent vector w T p (S 1 ) is the velocity vector α (0) of a differentiable parameterized curve α : ( ɛ, ɛ) V with α(0) = p. The curve β = ϕ α is such that β(0) = ϕ(p), and therefore β (0) is a vector of T ϕ(p) (S 2 ).

7 Proposition 2: In the discussion above, given w, the vector β (0) does not depend on the choice of α. The map dϕ p : T p (S 1 ) T ϕ(p) (S 2 ) defined by dϕ p (w) = β (0) is linear. This proposition shows that β (0) depends only on the map ϕ and the coordinates (u (0), v (0)) of w in the basis {x u, x v }. The linear map dϕ p is called the differential of ϕ at p S 1. In a similar way, we can define the differential of a differentiable function f : U S R at p U as a linear map df p : T p (S) R.

8 Proposition 3: If S 1 and S 2 are regular surfaces and ϕ : U S 1 S 2 is a differentiable mapping of an open set U S 1 such that the differential dϕ p of ϕ at p U is an isomorphism, then ϕ is a local diffeomorphism at p.

9 The First Fundamental Form Definition 1: The quadratic form I p (w) = < w, w > p = w 2 0 on T p (S) is called the first fundamental form of the regular surface S R 3 at p S. The first fundamental form is merely the expression of how the surface S inherits the natural inner product of R 3. And by knowing I p, we can treat metric questions on a regular surface without further references to the ambient space R 3.

10 In the basis of {x u, x v } associated to a parameterization x(u, v) at p, since a tangent vector w T p (S) is the tangent vector to a parameterized curve α(t) = x(u(t), v(t)), t ( ɛ, ɛ), with p = α(0) = x(u 0, v 0 ), we have I p (α (0)) = < α (0), α (0) > p = < x u u + x v v, x u u + x v v > p = < x u, x u > p (u ) 2 + 2< x u, x v > p u v + < x v, x v > p (v ) 2 = E(u ) 2 + 2F u v + G(v ) 2 where the values of the functions involved are computed for t = 0, and are the coefficients. E(u 0, v 0 ) = < x u, x u > p F (u 0, v 0 ) = < x u, x v > p G(u 0, v 0 ) = < x v, x v > p

11 Definition 2: Let R S be a bounded region of a regular surface contained in the coordinate neighborhood of the parameterization x : U R 2 S. The positive number A = = is called the area of R. x u x v dudv (EG F 2 ) dudv

12 Gauss Map In the study of regular curve, the rate of change of the tangent line to a curve C leads to an important geometry entity, the curvature. Here, we will try to measure how rapidly a surface S pulls away from the tangent plane T p (S) in a neighborhood of a point p S. This is equivalent to measuring the rate of change at p of a unit normal vector field N on a neighborhood of p, which is given by a linear map on T p (S).

13 Definition 1: Given a parameterization x : U R 2 S of a regular surface S at a point p S, a unit normal vector can be chosen at each point of x(u) by the rule N(q) = x u x v x u x v (q) This way, we have a differentiable map N : x(u) R 3 that associates to each q x(u) a unit normal vector N(q). More generally, if V S is an open set in S and N : V R 3 is a differentiable map which associates to each q V a unit normal vector at q, we say that N is a differentiable field of unit normal vectors on V.

14 Definition 2: A regular surface is orientable if it admits a differentiable field of unit normal vectors defined on the whole surface, and the choice of such a field N is called an orientation of S. An orientation N on S induces an orientation on each tangent plane T p (S), p S, as follows. Define a basis {v, w T p (S)} to be positive if < v w, N > is positive.

15 While every surface is locally orientable, not all surfaces admit a differentiable field of unit normal vectors defined on the whole surface (i.e., the Mobius strip).

16 Definition 3: Let S R 3 be a surface with an orientation N. The map N : S R 3 takes its values in the unit sphere S 2 = {(x, y, z) R 3 ; x 2 + y 2 + z 2 = 1} The map N : S S 2, thus defined, is called the Gauss map of S.

17 The linear map dn p : T p (S) T p (S) operates as follows. For each parameterized curve α(t) in S with α(0) = p, we consider the parameterized curve N α(t) = N(t) in the sphere S 2, this amounts to restricting the normal vector N to the curve α(t). The tangent vector N (0) = dn P (α (0)) is a vector in T p (S). It measures the rate of change of the normal vector N, restricted to the curve α(t), at t = 0. Thus, dn p measures how N pulls away from N(p) in a neighborhood of p.

18 Definition 4: A linear map A : V V is self-adjoint if < Av, w >=< v, Aw > for all v, w V. Proposition 1: The differential dn p : T p (S) T p (S) of the Gauss map is a self-adjoint linear map. This proposition allows us to associate to dn p a quadratic form Q in T p (S), given by Q(v) =< dn p (v), v >, v T p (S).

19 Definition 5: The quadratic form II p, defined in T p (S) by II p (v) = < dn p (v), v >, is called the second fundamental form of S at p. Definition 6: Let C be a regular curve in S passing through p S, k the curvature of C at p, and cosθ =< n, N >, where n is the normal vector to C and N is the normal vector to S at p. The number k n = kcosθ is then called the normal curvature of C subsets at p.

20

21 Consider a regular curve C S parameterized by α(s), where s is the arc length of C, and with α(0) = p. If we denote by N(s) the restriction of the normal vector N to the curve α(s), we have < N(s), α (s) >= 0. Hence, < N(s), α (s) >= < N (s), α(s) > Therefore II p (α (0)) = < dn p (α (0)), α (0) > = < N (0), α (0) > = < N(0), α (0) > = < N, kn > (p) = k n (p) In other words, the value of the second fundamental form II p for a unit vector v T p (S) is equal to the normal curvature of a regular curve passing through p and tangent to v.

22 Proposition 2 (Meusnier Theorem:) All curve lying on a surface S and having at a given point p S the same tangent line have at this point the same normal curvature.

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