MAT3009 Manifolds and Topology Semester 1 2020/1
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MAT3009/5/SEMR1 2020/1
Manifolds and Topology
Question 1
(a) In each of the following cases, determine whether or not the collection of subsets
τ S p(X) determines a topology on the given set X :
(i) X = {0, 1, 2, 3}, τ := {0, X, {0}, {1}, {0, 1}, {1, 2}, {2, 3}, {0, 1, 2}, {1, 2, 3}}; (ii) X = Z the integers, and A e τ if A = 0 or A contains an even integer.
(iii) X any set, A S X and
τ := {0, X, A, X \ A} .
(b) Let X = R and
τ := {0} u {A S R I R \ A countable} .
(By countable, we mean either finite (including empty) or bijective to the natural num- bers. In particular, R e τ .)
(i) Determine whether or not the topological space (R, τ ) is connected. Justify your answer.
(ii) Determine whether or not the topological space (R, τ ) is Hausdorff. Justify your
answer.
(iii) Let (an )neN be a sequence of elements of R. Show that if an converges to e e R with respect to the topology τ , then there exists N e N such that an = e for all n > N .
[Note: You are not required to show that τ defines a topology on R .]
(c) Let (X, τ ) be a Hausdorff topological space. Let A S X be a finite subset of X .
(i) Let x e X \ A. Show that there exists an open set Ux S X such that x e Ux , and Ux n A = 0 .
(ii) Show that A is closed.
Question 2
(a) Let M = RP3 , where RP3 = (R4 \ {0}) / ~ . Here ~ is the equivalence relation on
R4 \{0} defined by stating that x1 , x2 e R4 \{0} satisfy x2 ~ x1 if there exists λ e R\{0} such that x2 = λx1 . The equivalence class of a point x = (x1 , x2 , x3 , x4 ) e R4 \ {0} is denoted by [(x1 , x2 , x3 , x4 )] e RP3 .
We define charts (Ui , ϕi ), i = 1, 2, 3, 4, by
Ui = {[(x1 , x2 , x3 , x4 )] e RP3 I xi 0}, i = 1, 2, 3, 4,
with
ϕ 1 : U1 → R3 ; ϕ 1 ╱┌(x1 , x2 , x3 , x4 )┐、:= ╱ , , 、 , ϕ2 : U2 → R3 ; ϕ2 ╱┌(x1 , x2 , x3 , x4 )┐、:= ╱ , , 、 , ϕ3 : U3 → R3 ; ϕ3 ╱┌(x1 , x2 , x3 , x4 )┐、:= ╱ , , 、 . ϕ4 : U4 → R3 ; ϕ4 ╱┌(x1 , x2 , x3 , x4 )┐、:= ╱ , , 、 .
(i) Show that the charts (U2 , ϕ2 ) and (U3 , ϕ3 ) are compatible. Show that the charts (U1 , ϕ 1 ) and (U4 , ϕ4 ) are compatible.
[You may assume, without proof, that the maps ϕi , i = 1, . . . , 4, are bijections onto their images.]
(ii) Assuming that the charts (Ui , ϕi ) and (Uj , ϕj ) are compatible for all i and j, show
that RP3 is a differentiable manifold of dimension 3 .
(b) Let M = RP3 , with charts {(Ui , ϕi ), i = 1, . . . , 4} as in part (a). Let N = R, with
chart (V, ψ) with V = N = R and ψ : R → R the identity map (i.e. ψ(y) = y for all y e R).
Let f : RP3 → N be the map
x x f ╱┌(x1 , x2 , x3 , x4 )┐、:=
Show that the map f is well-defined and smooth.
Question 3
Let M = R3 , with chart (U, ϕ) given by U = M = R3 with ϕ : R3 → R3 the identity map and
Cartesian coordinates (x, y , z) e R3 . Let (a, b, c) e R3 be fixed, and f : M → M the map (x, y , z) 1→ f (x, y , z) := (a + x, b + y cosh a + z sinh a, c + z cosh a + y sinh a) .
(a) Show that, for all 女0 = (x0 , y0 , z0 ) e M , the tangent map T女0 f : T女0 M → Tf(女0) M takes the form
(T女0 f) ╱ │女0\ = │f(女0) ,
(T女0 f) ╱ │女0\ = cosh a │f(女0) + sinh a │f(女0) , (T女0 f) ╱ │女0\ = cosh a │f(女0) + sinh a │f(女0) .
(Please state, without proof, any results that you use concerning the coordinate rep-
resentation of the tangent map.)
(b) Show that the vector fields
e1 I女 := │女 ,
e2 I女 := cosh x │女 + sinh x │女 , e3 I女 := cosh x │女 + sinh x │女 ,
for all 女 = (x, y , z) e M have the property that
(T女0 f) ╱ei I女0 、= ei If(女0) , i = 1, 2, 3,
for all 女0 e M .
(c) Find the basis {σ1 , σ2 , σ3 } for T女0(*) M dual to the basis {e1 , e2 , e3 } for T女0 M given in part (b). Using the results of part (b), and stating any results that you use, how would you expect the pull-backs (T女0 f)* σ i , i = 1, 2, 3, to be related to the forms σ 1 , σ2 , σ3 ?
(d) Calculate explicitly the (1 , 1) tensor field σ i 8 ei , simplifying your result as much
as possible.
Question 4
Let M be a smooth manifold, Tk0 (M) the collection of smooth ╱k(0)、tensor fields on M , Ωk (M) the collection of smooth k-forms and 爻(M) the collection of smooth vector fields on M .
(a) Let α, β e Ω2 (M), and u1 , u2 , u3 , u4 e 爻(M). Find (α A β) (u1 , u2 , u3 , u4 ) in terms of
α(u1 , u2 ), β(u3 , u4 ), etc, simplifying your answer as much as possible. Show explicitly
that α A β = β A α .
(b) Let M = R3 with Cartesian coordinates (x, y , z). Let
α = a(x, y , z) dx + b(x, y , z) dy + c(x, y , z) dz e Ω1 (M),
where a(x, y , z), b(x, y , z), c(x, y , z) are smooth functions of (x, y , z) .
(i) Find dα .
(ii) Using the result of part (i), show explicitly that d (dα) = 0 .
(iii) Find α A dα. Show that if there exist smooth functions f (x, y , z), g(x, y , z) such
that α = f dg, then α A dα = 0 .
(c) Let ρ, t, x e R and r e R \ {0} be coordinates on a smooth four-dimensional manifold
M. Let
θ 1 = cosh ρ ╱ dt + r2 dx、, θ 2 = r2 cosh ρ dx, θ3 = dρ, θ4 = cosh ρ dr.
(i) Compute dθ i for i = 1 , 2, 3, 4, writing the resulting expressions as simply as pos- sible, entirely in terms of linear combinations of θj A θk .
(ii) Do there exist functions f, g such that θ 1 = f dg? Justify your answer.
2022-01-20