# B3.2 Geometry of Surfaces - Material for the year 2020-2021

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Part A Topology is recommended. Introduction to Manifolds would be useful but not essential. Also, B3.2 is helpful, but not essential, for B3.3 (Algebraic Curves).

16 lectures

### Assessment type:

- Written Examination

Different ways of thinking about surfaces (also called two-dimensional manifolds) are introduced in this course: first topological surfaces and then surfaces with extra structures which allow us to make sense of differentiable functions (`smooth surfaces'), holomorphic functions (`Riemann surfaces') and the measurement of lengths and areas ('Riemannian 2-manifolds').

These geometric structures interact in a fundamental way with the topology of the surfaces. A striking example of this is given by the Euler number, which is a manifestly topological quantity, but can be related to the total curvature, which at first glance depends on the geometry of the surface.

The course ends with an introduction to hyperbolic surfaces modelled on the hyperbolic plane, which gives us an example of a non-Euclidean geometry (that is, a geometry which meets all of Euclid's axioms except the axiom of parallels).

Students will be able to implement the classification of surfaces for simple constructions of topological surfaces such as planar models and connected sums; be able to relate the Euler characteristic to branching data for simple maps of Riemann surfaces; be able to describe the definition and use of Gaussian curvature; know the geodesics and isometries of the hyperbolic plane and their use in geometrical constructions.

The concept of a topological surface (or 2-manifold); examples, including polygons with pairs of sides identified. Orientation and the Euler characteristic. Classification theorem for compact surfaces (the proof will not be examined).

Riemann surfaces; examples, including the Riemann sphere, the quotient of the complex numbers by a lattice, and double coverings of the Riemann sphere. Holomorphic maps of Riemann surfaces and the Riemann-Hurwitz formula. Elliptic functions.

Smooth surfaces in Euclidean three-space and their first fundamental forms. The concept of a Riemannian 2-manifold; isometries; Gaussian curvature.

Geodesics. The Gauss-Bonnet Theorem (statement of local version and deduction of global version). Critical points of real-valued functions on compact surfaces.

The hyperbolic plane, its isometries and geodesics. Compact hyperbolic surfaces as Riemann surfaces and as surfaces of constant negative curvature.

- A. Pressley,
*Elementary Differential Geometry*, Springer Undergraduate Mathematics Series (Springer-Verlag, 2001). (Chapters 4-8 and 10-11.) - G. B. Segal,
*Geometry of Surfaces*, Mathematical Institute Notes (1989). - R. Earl,
*The Local Theory of Curves and Surfaces*, Mathematical Institute Notes (1999). - J. McCleary,
*Geometry from a Differentiable Viewpoint*, (Cambridge, 1997).

- P. A. Firby and C. E. Gardiner,
*Surface Topology*(Ellis Horwood, 1991) (Chapters 1-4 and 7). - F. Kirwan,
*Complex Algebraic Curves*, Student Texts 23 (London Mathematical Society, Cambridge, 1992) (Chapter 5.2 only). - B. O'Neill,
*Elementary Differential Geometry*(Academic Press, 1997). - M. P. do Carmo,
*Differential Geometry of Curves and Surfaces*(Dover, 2016)