Part III — Differential Geometry
                              Based on lectures by J. A. Ross
                                 Notes taken by Dexter Chua
                                   Michaelmas 2016
                     These notes are not endorsed by the lecturers, and I have modified them (often
                    significantly) after lectures. They are nowhere near accurate representations of what
                      was actually lectured, and in particular, all errors are almost surely mine.
                 This course is intended as an introduction to modern differential geometry. It can be
                 taken with a view to further studies in Geometry and Topology and should also be
                 suitable as a supplementary course if your main interests are, for instance in Analysis
                 or Mathematical Physics. A tentative syllabus is as follows.
                    • Local Analysis and Differential Manifolds. Definition and examples of manifolds,
                     smooth maps. Tangent vectors and vector fields, tangent bundle. Geometric
                     consequences of the implicit function theorem, submanifolds. Lie Groups.
                    • Vector Bundles. Structure group. The example of Hopf bundle. Bundle mor-
                     phisms and automorphisms. Exterior algebra of differential forms. Tensors.
                     Symplectic forms. Orientability of manifolds. Partitions of unity and integration
                     on manifolds, Stokes Theorem; de Rham cohomology. Lie derivative of tensors.
                     Connections on vector bundles and covariant derivatives: covariant exterior
                     derivative, curvature. Bianchi identity.
                    • Riemannian Geometry. Connections on the tangent bundle, torsion. Bianchi’s
                     identities for torsion free connections. Riemannian metrics, Levi-Civita con-
                     nection, Christoffel symbols, geodesics. Riemannian curvature tensor and its
                     symmetries, second Bianchi identity, sectional curvatures.
                 Pre-requisites
                 An essential pre-requisite is a working knowledge of linear algebra (including bilinear
                 forms) and multivariate calculus (e.g. differentiation and Taylor’s theorem in several
                 variables). Exposure to some of the ideas of classical differential geometry might also
                 be useful.
                                         1
                                 Contents                                                     III Differential Geometry
                                 Contents
                                 0 Introduction                                                                          3
                                 1 Manifolds                                                                             4
                                     1.1   Manifolds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       4
                                     1.2   Smooth functions and derivatives . . . . . . . . . . . . . . . . . .          8
                                     1.3   Bump functions and partitions of unity         . . . . . . . . . . . . . .   13
                                     1.4   Submanifolds . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       15
                                 2 Vector fields                                                                         19
                                     2.1   The tangent bundle . . . . . . . . . . . . . . . . . . . . . . . . . .       19
                                     2.2   Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      22
                                     2.3   Lie derivative . . . . . . . . . . . . . . . . . . . . . . . . . . . . .     27
                                 3 Lie groups                                                                           29
                                 4 Vector bundles                                                                       34
                                     4.1   Tensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      34
                                     4.2   Vector bundles . . . . . . . . . . . . . . . . . . . . . . . . . . . .       38
                                 5 Differential forms and de Rham cohomology                                             44
                                     5.1   Differential forms . . . . . . . . . . . . . . . . . . . . . . . . . . .      44
                                     5.2   De Rham cohomology . . . . . . . . . . . . . . . . . . . . . . . .           49
                                     5.3   Homological algebra and Mayer-Vietoris theorem . . . . . . . . .             53
                                 6 Integration                                                                          57
                                     6.1   Orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      57
                                     6.2   Integration    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   58
                                     6.3   Stokes Theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . .       61
                                 7 De Rham’s theorem*                                                                   67
                                 8 Connections                                                                          72
                                     8.1   Basic properties of connections . . . . . . . . . . . . . . . . . . .        72
                                     8.2   Geodesics and parallel transport       . . . . . . . . . . . . . . . . . .   76
                                     8.3   Riemannian connections . . . . . . . . . . . . . . . . . . . . . . .         78
                                     8.4   Curvature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      80
                                 Index                                                                                  86
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             0 Introduction          III Differential Geometry
             0  Introduction
             In differential geometry, the main object of study is a manifold. The motivation
             is as follows — from IA, we know well how to do calculus on Rn. We can talk
             about continuity, differentiable functions, derivatives etc. happily ever after.
               However, sometimes, we want to do calculus on things other than Rn. Say,
             we live on a sphere, namely the Earth. Does it make sense to “do calculus” on a
             sphere? Surely it does.
               The key insight is that these notions of differentiability, derivatives etc. are
             local properties. To know if a function is differentiable at a point p, we only need
             to know how the function behaves near p, and similarly such local information
             tells us how to compute derivatives. The reason we can do calculus on a sphere
             is because the sphere looks locally like Rn. Therefore, we can make sense of
             calculus on a sphere.
               Thus, what we want to do is to study calculus on things that look locally like
             Rn, and these are known as manifolds. Most of the time, our definitions from
             usual calculus on Rn transfer directly to manifolds. However, sometimes the
             global properties of our manifold will give us some new exciting things.
               In fact, we’ve already seen such things when we did IA Vector Calculus. If
             we have a vector field R3 → R3 whose curl vanishes everywhere, then we know
             it is the gradient of some function. However, if we consider such a vector field
             on R3 \{0} instead, then this is no longer true! Here the global topology of the
             space gives rise to interesting phenomena we do not see at a local level.
               When doing differential geometry, it is important to keep in mind that
             what we’ve learnt in vector calculus is actually a mess. R3 has a lot of special
             properties. Apart from being a topological space, it is also canonically a vector
             space, and in fact an inner product space. When we did vector calculus, these
             extra structure allowed us conflate many different concepts together. However,
             when we pass on to manifolds, we no longer have these identifications, and we
             have to be more careful.
                              3
                              1 Manifolds                                             III Differential Geometry
                              1     Manifolds
                              1.1     Manifolds
                              As mentioned in the introduction, manifolds are spaces that look locally like Rn.
                              This local identification with Rn is done via a chart.
                                  Many sources start off with a topological space and then add extra structure
                              to it, but we will be different and start with a bare set.
                              Definition(Chart). Achart (U,ϕ)onasetM isabijectionϕ : U → ϕ(U) ⊆ Rn,
                              where U ⊆ M and ϕ(U) is open.
                                  Achart (U,ϕ) is centered at p for p ∈ U if ϕ(p) = 0.
                                  Note that we do not require U to be open in M, or ϕ to be a homeomorphism,
                              because these concepts do not make sense! M is just a set, not a topological
                              space.
                                                                         p U
                                                                         ϕ
                                                                         ϕ(p)
                              With a chart, we can talk about things like continuity, differentiability by
                              identifying U with ϕ(U):
                              Definition (Smooth function). Let (U,ϕ) be a chart on M and f : M → R.
                                                        ∞                  −1
                              Wesay f is smooth or C      at p ∈ U if f ◦ ϕ    : ϕ(U) → R is smooth at ϕ(p) in
                              the usual sense.
                                                        Rn ⊇ϕ(U) ϕ−1 U          f    R
                                                         p U
                                                                                  f
                                                         ϕ
                                                                                   −1
                                                         ϕ(p)                f ◦ ϕ               R
                                                                       4