# Manual Algebraic Geometry

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Here hyperbolic geometry appears in both differential-geometric and metric form, where group actions on real-hyperbolic spaces, trees and Gromov-hyperbolic spaces serve as an important tool for studying both fundamental groups and geometric structure of compact Kaehler manifolds. Furthermore, real-hyperbolic geometry appeared as a tool in constructing complex-projective varieties with prescribed fundamental groups. Interesting connections and similarities between geometry of hyperbolic manifolds and geometric decompositions of 3-dimensional manifolds on one hand and study of projective varieties along the lines of the minimal model program.

Geometry of the compact quotients of complex balls, where algebraic and number-theoretic properties of lattices in PU n,1 are reflected in algebro-geometric properties of the ball quotients. Given a subset U of A n , can one recover the set of polynomials which generate it? If U is any subset of A n , define I U to be the set of all polynomials whose vanishing set contains U.

The answer to the first question is provided by introducing the Zariski topology , a topology on A n whose closed sets are the algebraic sets, and which directly reflects the algebraic structure of k [ A n ]. The answer to the second question is given by Hilbert's Nullstellensatz. In one of its forms, it says that I V S is the radical of the ideal generated by S.

In more abstract language, there is a Galois connection , giving rise to two closure operators ; they can be identified, and naturally play a basic role in the theory; the example is elaborated at Galois connection. For various reasons we may not always want to work with the entire ideal corresponding to an algebraic set U.

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Hilbert's basis theorem implies that ideals in k [ A n ] are always finitely generated. An algebraic set is called irreducible if it cannot be written as the union of two smaller algebraic sets.

An introduction to algebraic curves - Arithmetic and Geometry Math Foundations 76 - N J Wildberger

Any algebraic set is a finite union of irreducible algebraic sets and this decomposition is unique. Thus its elements are called the irreducible components of the algebraic set. An irreducible algebraic set is also called a variety. It turns out that an algebraic set is a variety if and only if it may be defined as the vanishing set of a prime ideal of the polynomial ring. Some authors do not make a clear distinction between algebraic sets and varieties and use irreducible variety to make the distinction when needed.

Just as continuous functions are the natural maps on topological spaces and smooth functions are the natural maps on differentiable manifolds , there is a natural class of functions on an algebraic set, called regular functions or polynomial functions. A regular function on an algebraic set V contained in A n is the restriction to V of a regular function on A n. For an algebraic set defined on the field of the complex numbers, the regular functions are smooth and even analytic.

## Rationality problems in algebraic geometry | American Inst. of Mathematics

It may seem unnaturally restrictive to require that a regular function always extend to the ambient space, but it is very similar to the situation in a normal topological space , where the Tietze extension theorem guarantees that a continuous function on a closed subset always extends to the ambient topological space.

Just as with the regular functions on affine space, the regular functions on V form a ring, which we denote by k [ V ]. This ring is called the coordinate ring of V. Since regular functions on V come from regular functions on A n , there is a relationship between the coordinate rings. Using regular functions from an affine variety to A 1 , we can define regular maps from one affine variety to another. First we will define a regular map from a variety into affine space: Let V be a variety contained in A n.

Choose m regular functions on V , and call them f 1 , In other words, each f i determines one coordinate of the range of f. The definition of the regular maps apply also to algebraic sets. The regular maps are also called morphisms , as they make the collection of all affine algebraic sets into a category , where the objects are the affine algebraic sets and the morphisms are the regular maps. The affine varieties is a subcategory of the category of the algebraic sets. This defines an equivalence of categories between the category of algebraic sets and the opposite category of the finitely generated reduced k -algebras.

This equivalence is one of the starting points of scheme theory. In contrast to the preceding sections, this section concerns only varieties and not algebraic sets. On the other hand, the definitions extend naturally to projective varieties next section , as an affine variety and its projective completion have the same field of functions.

If V is an affine variety, its coordinate ring is an integral domain and has thus a field of fractions which is denoted k V and called the field of the rational functions on V or, shortly, the function field of V. Its elements are the restrictions to V of the rational functions over the affine space containing V.

The domain of a rational function f is not V but the complement of the subvariety a hypersurface where the denominator of f vanishes.

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As with regular maps, one may define a rational map from a variety V to a variety V '. As with the regular maps, the rational maps from V to V ' may be identified to the field homomorphisms from k V ' to k V. Two affine varieties are birationally equivalent if there are two rational functions between them which are inverse one to the other in the regions where both are defined.

Equivalently, they are birationally equivalent if their function fields are isomorphic. An affine variety is a rational variety if it is birationally equivalent to an affine space. This means that the variety admits a rational parameterization. The problem of resolution of singularities is to know if every algebraic variety is birationally equivalent to a variety whose projective completion is nonsingular see also smooth completion.

It was solved in the affirmative in characteristic 0 by Heisuke Hironaka in and is yet unsolved in finite characteristic. Just as the formulas for the roots of second, third, and fourth degree polynomials suggest extending real numbers to the more algebraically complete setting of the complex numbers, many properties of algebraic varieties suggest extending affine space to a more geometrically complete projective space.

If we draw it, we get a parabola.

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• As x goes to negative infinity, the slope of the same line goes to negative infinity. This is a cubic curve. But unlike before, as x goes to negative infinity, the slope of the same line goes to positive infinity as well; the exact opposite of the parabola. The consideration of the projective completion of the two curves, which is their prolongation "at infinity" in the projective plane , allows us to quantify this difference: the point at infinity of the parabola is a regular point , whose tangent is the line at infinity , while the point at infinity of the cubic curve is a cusp.

Also, both curves are rational, as they are parameterized by x , and the Riemann-Roch theorem implies that the cubic curve must have a singularity, which must be at infinity, as all its points in the affine space are regular.

## Algebraic geometry

Thus many of the properties of algebraic varieties, including birational equivalence and all the topological properties, depend on the behavior "at infinity" and so it is natural to study the varieties in projective space. For these reasons, projective space plays a fundamental role in algebraic geometry.

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In this case, one says that the polynomial vanishes at the corresponding point of P n. This allows us to define a projective algebraic set in P n as the set V f 1 , Like for affine algebraic sets, there is a bijection between the projective algebraic sets and the reduced homogeneous ideals which define them. The projective varieties are the projective algebraic sets whose defining ideal is prime.

Every projective algebraic set may be uniquely decomposed into a finite union of projective varieties. The only regular functions which may be defined properly on a projective variety are the constant functions.

## ALGEBRAIC GEOMETRY AND NUMBER THEORY SEMINAR

Thus this notion is not used in projective situations. On the other hand, the field of the rational functions or function field is a useful notion, which, similarly to the affine case, is defined as the set of the quotients of two homogeneous elements of the same degree in the homogeneous coordinate ring. The fact that the field of the real numbers is an ordered field cannot be ignored in such a study. It follows that real algebraic geometry is not only the study of the real algebraic varieties, but has been generalized to the study of the semi-algebraic sets , which are the solutions of systems of polynomial equations and polynomial inequalities.

One of the challenging problems of real algebraic geometry is the unsolved Hilbert's sixteenth problem : Decide which respective positions are possible for the ovals of a nonsingular plane curve of degree 8. At this meeting,. Since then, most results in this area are related to one or several of these items either by using or improving one of these algorithms, or by finding algorithms whose complexity is simply exponential in the number of the variables. A body of mathematical theory complementary to symbolic methods called numerical algebraic geometry has been developed over the last several decades.

The main computational method is homotopy continuation. This supports, for example, a model of floating point computation for solving problems of algebraic geometry. In fact they may contain, in the worst case, polynomials whose degree is doubly exponential in the number of variables and a number of polynomials which is also doubly exponential. However, this is only a worst case complexity, and the complexity bound of Lazard's algorithm of may frequently apply.

It follows that the best implementations allow one to compute almost routinely with algebraic sets of degree more than CAD is an algorithm which was introduced in by G. Collins to implement with an acceptable complexity the Tarski—Seidenberg theorem on quantifier elimination over the real numbers. This theorem concerns the formulas of the first-order logic whose atomic formulas are polynomial equalities or inequalities between polynomials with real coefficients.

The complexity of CAD is doubly exponential in the number of variables.

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