# C3.4 Algebraic Geometry (2019-2020)

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A3 Rings and Modules is essential. In particular, a solid understanding of the definitions of: field, ring, ideal (prime ideal, maximal ideal), integral domain, field of fractions, module, polynomial ring, irreducible and prime elements, zero divisor, unique factorisation domain, quotient of a ring by an ideal, homomorphisms and isomorphisms. Basic results such as the first isomorphism theorem for rings and the Chinese Remainder Theorem.

B3.3 Algebraic Curves is useful but not essential. Projective spaces and homogeneous coordinates will be defined in C3.4, but a working knowledge of them would be useful. There is some overlap of topics, as B3.3 studies the algebraic geometry of one-dimensional varieties.

B2.2 Commutative Algebra is useful but not essential. There is a substantial overlap of topics with this course, but in C3.4 these topics are rephrased in terms of geometrical properties.

Courses closely related to C3.4 include C2.2 Homological Algebra, C2.7 Category Theory, C3.7 Elliptic Curves, C2.6 Introduction to Schemes; and partly related to: C3.1 Algebraic Topology, C3.3 Differentiable Manifolds, C3.5 Lie Groups.

16 lectures.

### Assessment type:

- Written Examination

Algebraic geometry is the study of algebraic varieties: an algebraic variety is roughly speaking, a locus defined by polynomial equations. One of the advantages of algebraic geometry is that it is purely algebraically defined and applied to any field, including fields of finite characteristic. It is geometry based on algebra rather than calculus, but over the real or complex numbers it provides a rich source of examples and inspiration to other areas of geometry.

Affine algebraic varieties, the Zariski topology, morphisms of affine varieties. Irreducible varieties.

Projective space. Projective varieties, affine cones over projective varieties. The Zariski topology on projective varieties. The projective closure of affine variety. Morphisms of projective varieties. Projective equivalence.

Veronese morphism: definition, examples. Veronese morphisms are isomorphisms onto their image; statement, and proof in simple cases. Subvarieties of Veronese varieties. Segre maps and products of varieties.

Coordinate rings. Hilbert's Nullstellensatz. Correspondence between affine varieties (and morphisms between them) and finitely generated reduced $k$-algebras (and morphisms between them). Graded rings and homogeneous ideals. Homogeneous coordinate rings.

Discrete invariants of projective varieties: degree, dimension, Hilbert function. Statement of theorem defining Hilbert polynomial.

Quasi-projective varieties, and morphisms between them. The Zariski topology has a basis of affine open subsets. Rings of regular functions on open subsets and points of quasi-projective varieties. The ring of regular functions on an affine variety is the coordinate ring. Localisation and relationship with rings of regular functions.

Tangent space and smooth points. The singular locus is a closed subvariety. Algebraic re-formulation of the tangent space. Differentiable maps between tangent spaces.

Function fields of irreducible quasi-projective varieties. Rational maps between irreducible varieties, and composition of rational maps. Birational equivalence. Correspondence between dominant rational maps and homomorphisms of function fields. Blow-ups: of affine space at a point, of subvarieties of affine space, and of general quasi-projective varieties along general subvarieties. Statement of Hironaka's Desingularisation Theorem. Every irreducible variety is birational to a hypersurface. Re-formulation of dimension. Smooth points are a dense open subset.

- KE Smith et al,
*An Invitation to Algebraic Geometry*, (Springer 2000), Chapters 1-8.

*Please note that e-book versions of many books in the reading lists can be found on SOLO and ORLO.*

- M Reid,
*Undergraduate Algebraic Geometry*, LMS Student Texts 12, (Cambridge 1988). - K Hulek,
*Elementary Algebraic Geometry*, Student Mathematical Library 20. (American Mathematical Society, 2003). - A Gathmann,
*Algebraic Geometry lecture notes* - Shafarevich,
*Basic Algebraic Geometry 1*, (Springer, 1994). - D Mumford,
*The Red Book of Varieties and Schemes*, (Springer, 2009).