# Introduction and Basic Definitions

We introduce vector bundles, sections and other basic definitions before building up to the Chern classes of line bundles through a sequence of examples.

## What are Chern classes?[edit | edit source]

It is usually a difficult problem to classify all non-isomorphic vector bundles over a given fixed base space. The idea of characteristic classes is to provide a topological invariant which allows us to distinguish some classes of vector bundles. Characteristic classes are elements of the cohomology groups of the base space. Chern classes are particular characteristic classes which we associate to complex vector bundles. They are a sequence of functions assigning to each complex vector bundle a class depending only on the isomorphism type of (and are the unique such sequence satisfying a list of properties which we'll list towards the end). The Chern classes are formally akin to the Stiefel-Whitney classes, which are defined for real vector bundles. The Stiefel-Whitney classes take coefficients in .

## Vector Bundles[edit | edit source]

In the course of this lecture we denote by , a smooth manifold of real dimension .
A **rank smooth complex vector bundle** is a map together with a complex vector space structure on the **fibres** for each such that we have the following local triviality condition: for each there exists an open neighbourhood of such that there is a diffeomorphism which is a vector space isomorphism.
We call the **local trivialisation** of the vector bundle at . We call the **total space** and call the **base space** of the vector bundle. For convenience, we refer to the bundle simply as with all of the other data above quietly suppressed.

A **smooth section** of the vector bundle is a smooth right inverse of . The **trivial bundle** over is the vector bundle . Any vector bundle is said to be **trivial** if it is isomorphic to the trivial bundle.

**Exercise 4.1**

Show how to put the real line bundle structure on the open Mobius band

**Exercise 4.2**

Show that is isotopic in to the trivial embedding of the cylinder.

**Remark**

Bundles on contractible manifolds are trivial

Sketch proof: After trivialising at some fibre lying over , we extend the trivialisation along the homotopy that contracts M to using a version of Tietze's Extension theorem. We then lift the homotopy to E so that it covers the homotopy on .