Multivariable Analysis Spring 2023 Homework 5
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Multivariable Analysis Spring 2023
Homework 5, due 11:59pm Tuesay, May 2, 2023
This assignment assumes that you are familiar with the definition and properties of the dot product on Rm . In particular, an ordered pair of vectors (v1 ,v2 ) is called orthonormal if
v1 · v1 = v2 · v2 = 1
v1 ··· v2 = 0
1. (5 points) If F : M → N is a smooth map from the manifold M to the manifold N , show that at each p ∈ M, the ranks of the pushforward
F* : Tp M → TF(p)N
and the pullback
F* : TF(*)(p)N → T*pM
are equal.
2. (5 points) Let e1 ,e2 be vector fields on a 2-manifold S such that for each p ∈ S , (e1 (p),e2 (p)) is a basis of Tp S . Let ω 1 ,ω 2 be 1-dorms such that (ω1 (p),ω2 (p)) is the dual basis to (e1 (p),e2 (p)). Show that there is a unique 1-form ω2(1) such that, if we denote ω 1(2) = −ω2(1), then
dω 1 + ω2(1) V ω 2 = 0
dω 2 + ω1(2) V ω 1 = 0
3. The sphere of radius 1 centered at the origin is
S = {(x,y,z) ∈ R3 : f(x,y,z)) = 0} ⊂ R3 ,
where
f(v) = x2 + y2 + z2 − 1.
Recall that stereographic projection from the north pole N = (0, 0, 1) is a coordinate map
Φ : R2 → S/{N}
(u,v) '→ ( , , ) .
3.1. (5 points) The tangent space at each v = (x,y,z) ∈ S is a linear subspace of R3 .
Find it for each v ∈ S .
3.2. (5 points) Let (?1 ,?1 ) denote the standard basis in R2 and, at each (u,v) ∈ R2 ,
calculate Φ* ?1 , Φ* ?2 .
3.3. (5 points) Calculate Φ* ?1 · Φ* ?1 , Φ* ?2 · Φ* ?2 , Φ* ?1 · Φ* ?2 .
3.4. (5 points) Find an pair, not necessarily orthonormal, of vectors e1 ,e2 ∈ R2 such
that
(f1 ,f2 ) = (Φ* e1 , Φ* e2 )
is an orthonormal basis of Tp S .
3.5. (5 points) Denote the dual basis of (?1 ,?2 ) by (du,dv). Find 1-forms ω 1 ,ω 2 such
that, at each (u,v) ∈ R2 , (ω1 ,ω 2 ) is the dual basis of (e1 ,e2 ).
3.6. (5 points) Find the 1-form ω2(1) defined in problem 2.
3.7. (5 points) The Gauss curvature of S is defined to be the function K such that dω2(1) = Kω 1 ∧ ω 2 .
Find K .
3.8. (5 points) Compute, using the standard orientation and polar coordinates on ,
\R2 Φ* (dω2(1)).
3.9. (5 points) Stokes’ Theorem says that if θ is a (m − 1)-form on a compact m-
manifold M, then
\M dθ = 0.
Explain why, despite the fact that S is a compact manifold, the answer to the previous problem does not contradict this theorem.
3.10. (5 points) Given −1 ≤ h < 1, let
Dh = {(x,y,z) ∈ S : z ≤ h}.
The area of Dh is
\Dh ω 1 V ω 2 .
Compute the area of Dh .
3.11. (5 points) Compute directly the integral
\∂Dh ω2(1) .
Since dω2(1) = ω 1 ∧ ω 2 , you should get the same value as the previous problem.
2023-05-04