Algebraic variety

Algebraic varieties are the central objects of study in algebraic geometry. Classically, an algebraic variety is defined as the set of solutions of a system of polynomial equations over the real or complex numbers. Modern definitions generalize this concept in several different ways, while attempting to preserve the geometric intuition behind the original definition.:58Conventions regarding the definition of an algebraic variety differ slightly. For example, some definitions require an algebraic variety to be irreducible, which means that it is not the union of two smaller sets that are closed in the Zariski topology. Under this definition, non-irreducible algebraic varieties are called algebraic sets. Other conventions do not require irreducibility; the concept of an algebraic variety is similar to that of an analytic manifold. An important difference is that an algebraic variety may have singular points, while a manifold cannot; the fundamental theorem of algebra establishes a link between algebra and geometry by showing that a monic polynomial in one variable with complex number coefficients is determined by the set of its roots in the complex plane.

Generalizing this result, Hilbert's Nullstellensatz provides a fundamental correspondence between ideals of polynomial rings and algebraic sets. Using the Nullstellensatz and related results, mathematicians have established a strong correspondence between questions on algebraic sets and questions of ring theory; this correspondence is a defining feature of algebraic geometry. An affine variety over an algebraically closed field is conceptually the easiest type of variety to define, which will be done in this section. Next, one can define quasi-projective varieties in a similar way; the most general definition of a variety is obtained by patching together smaller quasi-projective varieties. It is not obvious that one can construct genuinely new examples of varieties in this way, but Nagata gave an example of such a new variety in the 1950s. For an algebraically closed field K and a natural number n, let An be affine n-space over K; the polynomials f in the ring K can be viewed as K-valued functions on An by evaluating f at the points in An, i.e. by choosing values in K for each xi.

For each set S of polynomials in K, define the zero-locus Z to be the set of points in An on which the functions in S vanish, to say Z =. A subset V of An is called an affine algebraic set if V = Z for some S.:2 A nonempty affine algebraic set V is called irreducible if it cannot be written as the union of two proper algebraic subsets.:3 An irreducible affine algebraic set is called an affine variety.:3 Affine varieties can be given a natural topology by declaring the closed sets to be the affine algebraic sets. This topology is called the Zariski topology.:2Given a subset V of An, we define I to be the ideal of all polynomial functions vanishing on V: I =. For any affine algebraic set V, the coordinate ring or structure ring of V is the quotient of the polynomial ring by this ideal.:4 Let k be an algebraically closed field and let Pn be the projective n-space over k. Let f in k be a homogeneous polynomial of degree d, it is not well-defined to evaluate f on points in Pn in homogeneous coordinates.

However, because f is homogeneous, meaning that f = λd f , it does make sense to ask whether f vanishes at a point. For each set S of homogeneous polynomials, define the zero-locus of S to be the set of points in Pn on which the functions in S vanish: Z =. A subset V of Pn is called a projective algebraic set if V = Z for some S.:9 An irreducible projective algebraic set is called a projective variety.:10Projective varieties are equipped with the Zariski topology by declaring all algebraic sets to be closed. Given a subset V of Pn, let I be the ideal generated by all homogeneous polynomials vanishing on V. For any projective algebraic set V, the coordinate ring of V is the quotient of the polynomial ring by this ideal.:10A quasi-projective variety is a Zariski open subset of a projective variety. Notice that every affine variety is quasi-projective. Notice that the complement of an algebraic set in an affine variety is a quasi-projective variety. In classical algebraic geometry, a

Algebraic geometry

Algebraic geometry is a branch of mathematics, classically studying zeros of multivariate polynomials. Modern algebraic geometry is based on the use of abstract algebraic techniques from commutative algebra, for solving geometrical problems about these sets of zeros; the fundamental objects of study in algebraic geometry are algebraic varieties, which are geometric manifestations of solutions of systems of polynomial equations. Examples of the most studied classes of algebraic varieties are: plane algebraic curves, which include lines, parabolas, hyperbolas, cubic curves like elliptic curves, quartic curves like lemniscates and Cassini ovals. A point of the plane belongs to an algebraic curve if its coordinates satisfy a given polynomial equation. Basic questions involve the study of the points of special interest like the singular points, the inflection points and the points at infinity. More advanced questions involve the topology of the curve and relations between the curves given by different equations.

Algebraic geometry occupies a central place in modern mathematics and has multiple conceptual connections with such diverse fields as complex analysis and number theory. A study of systems of polynomial equations in several variables, the subject of algebraic geometry starts where equation solving leaves off, it becomes more important to understand the intrinsic properties of the totality of solutions of a system of equations, than to find a specific solution. In the 20th century, algebraic geometry split into several subareas; the mainstream of algebraic geometry is devoted to the study of the complex points of the algebraic varieties and more to the points with coordinates in an algebraically closed field. Real algebraic geometry is the study of the real points of an algebraic variety. Diophantine geometry and, more arithmetic geometry is the study of the points of an algebraic variety with coordinates in fields that are not algebraically closed and occur in algebraic number theory, such as the field of rational numbers, number fields, finite fields, function fields, p-adic fields.

A large part of singularity theory is devoted to the singularities of algebraic varieties. Computational algebraic geometry is an area that has emerged at the intersection of algebraic geometry and computer algebra, with the rise of computers, it consists of algorithm design and software development for the study of properties of explicitly given algebraic varieties. Much of the development of the mainstream of algebraic geometry in the 20th century occurred within an abstract algebraic framework, with increasing emphasis being placed on "intrinsic" properties of algebraic varieties not dependent on any particular way of embedding the variety in an ambient coordinate space. One key achievement of this abstract algebraic geometry is Grothendieck's scheme theory which allows one to use sheaf theory to study algebraic varieties in a way, similar to its use in the study of differential and analytic manifolds; this is obtained by extending the notion of point: In classical algebraic geometry, a point of an affine variety may be identified, through Hilbert's Nullstellensatz, with a maximal ideal of the coordinate ring, while the points of the corresponding affine scheme are all prime ideals of this ring.

This means that a point of such a scheme may be either a subvariety. This approach enables a unification of the language and the tools of classical algebraic geometry concerned with complex points, of algebraic number theory. Wiles' proof of the longstanding conjecture called Fermat's last theorem is an example of the power of this approach. In classical algebraic geometry, the main objects of interest are the vanishing sets of collections of polynomials, meaning the set of all points that satisfy one or more polynomial equations. For instance, the two-dimensional sphere of radius 1 in three-dimensional Euclidean space R3 could be defined as the set of all points with x 2 + y 2 + z 2 − 1 = 0. A "slanted" circle in R3 can be defined as the set of all points which satisfy the two polynomial equations x 2 + y 2 + z 2 − 1 = 0, x + y + z = 0. First we start with a field k. In classical algebraic geometry, this field was always the complex numbers C, but many of the same results are true if we assume only that k is algebraically closed.

We consider the affine space of dimension n over denoted An. When one fixes a coordinate system, one may identify An with kn; the purpose of not working with kn is to emphasize that one "forgets" the vector space structure that kn carries. A function f: An → A1 is said to be polynomial if it can be written as a polynomial, that is, if there is a polynomial p in k such that f = p for every point M with coordinates in An; the property of a function to be polynomial does not depend on the choice of a coordinate system in An. When a coordinate system is chosen, the regular functions on the affine n-space may be identified with the ring of polynomial functions in n variables over k. Therefore, the set of the

Serge Lang

Serge Lang was a French-American mathematician and activist who taught at Yale University for most of his career. He is known for his work in number theory and for his mathematics textbooks, including the influential Algebra, he was a member of the Bourbaki group. As an activist, he campaigned against the nomination of the political scientist Samuel P. Huntington to the National Academies of Science, descended into AIDS denialism, claiming that HIV had not been proven to cause AIDS and protesting Yale's research into HIV/AIDS. Lang was born in Saint-Germain-en-Laye, close to Paris, in 1927, he had a twin brother who became a sister who became an actress. Lang moved with his family to California as a teenager, where he graduated in 1943 from Beverly Hills High School, he subsequently graduated from the California Institute of Technology in 1946, received a doctorate from Princeton University in 1951. He held faculty positions at the University of Chicago, Columbia University, Yale University. Lang studied under Emil Artin at Princeton University, writing his thesis on quasi-algebraic closure, worked on the geometric analogues of class field theory and diophantine geometry.

He moved into diophantine approximation and transcendental number theory, proving the Schneider–Lang theorem. A break in research while he was involved in trying to meet 1960s student activism halfway caused him difficulties in picking up the threads afterwards, he wrote on modular forms and modular units, the idea of a'distribution' on a profinite group, value distribution theory. He made a number of conjectures in diophantine geometry: Mordell–Lang conjecture, Bombieri–Lang conjecture, Lang–Trotter conjecture, the Lang conjecture on analytically hyperbolic varieties, he introduced the Lang map, the Katz–Lang finiteness theorem, the Lang–Steinberg theorem in algebraic groups. Lang was a prolific writer of mathematical texts completing one on his summer vacation. Most are at the graduate level, he wrote calculus texts and prepared a book on group cohomology for Bourbaki. Lang's Algebra, a graduate-level introduction to abstract algebra, was a influential text that ran through numerous updated editions.

His Steele prize citation stated, "Lang's Algebra changed the way graduate algebra is taught... It has affected all subsequent graduate-level algebra books." It contained ideas of Artin. Lang was noted for his eagerness for contact with students, he was described as a passionate teacher who would throw chalk at students who he believed were not paying attention. One of his colleagues recalled: "He would rave in front of his students, he would say,'Our two aims are truth and clarity, to achieve these I will shout in class.'" He won a Leroy P. Steele Prize for Mathematical Exposition from the American Mathematical Society. In 1960, he won the sixth Frank Nelson Cole Prize in Algebra for his paper Unramified class field theory over function fields in several variables. Lang spent much of his professional time engaged in political activism, he was a staunch socialist and active in opposition to the Vietnam War, volunteering for the 1966 anti-war campaign of Robert Scheer. Lang quit his position at Columbia in 1971 in protest over the university's treatment of anti-war protesters.

Lang engaged in several efforts to challenge anyone he believed was spreading misinformation or misusing science or mathematics to further their own goals. He attacked the 1977 Survey of the American Professoriate, an opinion questionnaire that Seymour Martin Lipset and E. C. Ladd had sent to thousands of college professors in the United States, accusing it of containing numerous biased and loaded questions; this led to a public and acrimonious conflict. In 1986, Lang mounted what the New York Times described as a "one-man challenge" against the nomination of political scientist Samuel P. Huntington to the National Academy of Sciences. Lang described Huntington's research, in particular his use of mathematical equations to demonstrate that South Africa was a "satisfied society", as "pseudoscience", arguing that it gave "the illusion of science without any of its substance." Despite support for Huntington from the Academy's social and behavioral scientists, Lang's challenge was successful, Huntington was twice rejected for Academy membership.

Huntington's supporters argued that Lang's opposition was political rather than scientific in nature. Lang kept his political correspondence and related documentation in extensive "files", he would send letters or publish articles, wait for responses, engage the writers in further correspondence, collect all these writings together and point out what he considered contradictions. He mailed these files to people he considered important, his extensive file criticizing Nobel laureate David Baltimore was published in the journal Ethics and Behaviour in January 1993. Lang fought the decision by Yale University to hire Daniel Kevles, a historian of science, because Lang disagreed with Kevles' analysis in The Baltimore Case. Lang's most controversial political stance was as an AIDS denialist.

Terence Tao

Terence Chi-Shen Tao is an Australian-American mathematician who has worked in various areas of mathematics. He focuses on harmonic analysis, partial differential equations, algebraic combinatorics, arithmetic combinatorics, geometric combinatorics, compressed sensing and analytic number theory; as of 2015, he holds the James and Carol Collins chair in mathematics at the University of California, Los Angeles. Tao was the 2014 Breakthrough Prize in Mathematics, he is the second ethnic Chinese person to win the Fields medal after Shing-Tung Yau, the first Australian mathematician to win the Fields medal. Tao's father, Dr. Billy Tao, was a pediatrician, born in Shanghai and earned his medical degree from the University of Hong Kong in 1969. Tao's mother, Grace, is from Hong Kong, she was a secondary school teacher of physics in Hong Kong. Billy and Grace met as students at the University of Hong Kong, they emigrated from Hong Kong to Australia. Tao has two brothers and Trevor, living in Australia. Both represented Australia at the International Mathematical Olympiad.

Tao's wife, Laura, is an engineer at NASA's Jet Propulsion Laboratory. They live with their daughter in Los Angeles, California. Tao exhibited extraordinary mathematical abilities from an early age, attending university-level mathematics courses at the age of 9, he and Lenhard Ng are the only two children in the history of the Johns Hopkins' Study of Exceptional Talent program to have achieved a score of 700 or greater on the SAT math section while just nine years old. Tao was the youngest participant to date in the International Mathematical Olympiad, first competing at the age of ten, he remains the youngest winner of each of the three medals in the Olympiad's history, winning the gold medal shortly after his thirteenth birthday. At age 14, Tao attended the Research Science Institute; when he was 15, he published his first assistant paper. In 1991, he received his bachelor's and master's degrees at the age of 16 from Flinders University under Garth Gaudry. In 1992, he won a Postgraduate Fulbright Scholarship to undertake research in Mathematics at Princeton University in the United States.

From 1992 to 1996, Tao was a graduate student at Princeton University under the direction of Elias Stein, receiving his PhD at the age of 21. He joined the faculty of the University of California, Los Angeles. In 1999, when he was 24, he was promoted to full professor at UCLA and remains the youngest person appointed to that rank by the institution. Within the field of mathematics, Tao is known for his collaboration with Ben J. Green of Oxford University. Known for his collaborative mindset, by 2006, Tao had worked with over 30 others in his discoveries, reaching 68 co-authors by October 2015. In a book review, the mathematician Timothy Gowers remarked on Tao's accomplishments: Tao's mathematical knowledge has an extraordinary combination of breadth and depth: he can write confidently and authoritatively on topics as diverse as partial differential equations, analytic number theory, the geometry of 3-manifolds, nonstandard analysis, group theory, model theory, quantum mechanics, ergodic theory, harmonic analysis, image processing, functional analysis, many others.

Some of these are areas. Others are areas that he appears to understand at the deep intuitive level of an expert despite not working in those areas. How he does all this, as well as writing papers and books at a prodigious rate, is a complete mystery, it has been said that David Hilbert was the last person to know all of mathematics, but it is not easy to find gaps in Tao's knowledge, if you do you may well find that the gaps have been filled a year later. Tao has won numerous awards over the years, he is a Fellow of the Royal Society, the Australian Academy of Science, the National Academy of Sciences, the American Academy of Arts and Sciences, the American Mathematical Society. In 2006 he received the Fields Medal "for his contributions to partial differential equations, harmonic analysis and additive number theory", was awarded the MacArthur Fellowship, he has been featured in The New York Times, CNN, USA Today, Popular Science, many other media outlets. By 2016, Tao had published about 17 books.

He has an Erdős number of 2. In 2018, Tao proved Bounding the de Bruijn-Newman constant. In 2004, Ben Green and Tao released a preprint proving; this theorem states. The New York Times described it this way: In 2004, Dr. Tao, along with Ben Green, a mathematician now at the University of Cambridge in England, solved a problem related to the Twin Prime Conjecture by looking at prime number progressions—series of numbers spaced. Dr. Tao and Dr. Green proved that it is always possible to find, somewhere in the infinity of integers, a progression of prime numbers of equal spacing and any length. For this and other work Tao was awarded the Australian Mathematical Society Medal of 2004, he was awarded a Fields Medal in August 2006 at the 25th International Cong

Faltings's theorem

In number theory, the Mordell conjecture is the conjecture made by Mordell that a curve of genus greater than 1 over the field Q of rational numbers has only finitely many rational points. In 1983 it was proved by Gerd Faltings, is now known as Faltings's theorem; the conjecture was generalized by replacing Q by any number field. Let C be a non-singular algebraic curve of genus g over Q; the set of rational points on C may be determined as follows: Case g = 0: no points or infinitely many. Case g = 1: no points, or C is an elliptic curve and its rational points form a finitely generated abelian group. Moreover, Mazur's torsion theorem restricts the structure of the torsion subgroup. Case g > 1: according to the Mordell conjecture, now Faltings's theorem, C has only a finite number of rational points. Faltings's original proof used the known reduction to a case of the Tate conjecture, a number of tools from algebraic geometry, including the theory of Néron models. A different proof, based on diophantine approximation, was found by Vojta.

A more elementary variant of Vojta's proof was given by Bombieri. Faltings's 1983 paper had as consequences a number of statements, conjectured: The Mordell conjecture that a curve of genus greater than 1 over a number field has only finitely many rational points; the reduction of the Mordell conjecture to the Shafarevich conjecture was due to A. N. Paršin. A sample application of Faltings's theorem is to a weak form of Fermat's Last Theorem: for any fixed n > 4 there are at most finitely many primitive integer solutions to an + bn = cn, since for such n the curve xn + yn = 1 has genus greater than 1. Because of the Mordell–Weil theorem, Faltings's theorem can be reformulated as a statement about the intersection of a curve C with a finitely generated subgroup Γ of an abelian variety A. Generalizing by replacing C by an arbitrary subvariety of A and Γ by an arbitrary finite-rank subgroup of A leads to the Mordell–Lang conjecture, proved by Faltings. Another higher-dimensional generalization of Faltings's theorem is the Bombieri–Lang conjecture that if X is a pseudo-canonical variety over a number field k X is not Zariski dense in X.

More general conjectures have been put forth by Paul Vojta. The Mordell conjecture for function fields was proved by Manin and by Grauert. In 1990, Coleman found and fixed a gap in Manin's proof. Bombieri, Enrico. "The Mordell conjecture revisited". Ann. Scuola Norm. Sup. Pisa Cl. Sci. 17: 615–640. MR 1093712. Coleman, Robert F.. "Manin's proof of the Mordell conjecture over function fields". L'Enseignement Mathématique. Revue Internationale. IIe Série. 36: 393–427. ISSN 0013-8584. MR 1096426. Archived from the original on 2011-10-02. Cornell, Gary. Arithmetic geometry. Papers from the conference held at the University of Connecticut, Connecticut, July 30 – August 10, 1984. New York: Springer-Verlag. Doi:10.1007/978-1-4613-8655-1. ISBN 0-387-96311-1. MR 0861969. → Contains an English translation of Faltings Faltings, Gerd. "Endlichkeitssätze für abelsche Varietäten über Zahlkörpern". Inventiones Mathematicae. 73: 349–366. Doi:10.1007/BF01388432. MR 0718935. Faltings, Gerd. "Erratum: Endlichkeitssätze für abelsche Varietäten über Zahlkörpern".

Inventiones Mathematicae. 75: 381. Doi:10.1007/BF01388572. MR 0732554. Faltings, Gerd. "Diophantine approximation on abelian varieties". Ann. of Math. 133: 549–576. Doi:10.2307/2944319. MR 1109353. Faltings, Gerd. "The general case of S. Lang's conjecture". In Cristante, Valentino. Barsotti Symposium in Algebraic Geometry. Papers from the symposium held in Abano Terme, June 24–27, 1991. Perspectives in Mathematics. San Diego, CA: Academic Press, Inc. ISBN 0-12-197270-4. MR 1307396. Grauert, Hans. "Mordells Vermutung über rationale Punkte auf algebraischen Kurven und Funktionenkörper". Publications Mathématiques de l'IHÉS: 131–149. ISSN 1618-1913. MR 0222087. Hindry, Marc. Diophantine geometry. Graduate Texts in Mathematics. 201. New York: Springer-Verlag. Doi:10.1007/978-1-4612-1210-2. ISBN 0-387-98981-1. MR 1745599. → Gives Vojta's proof of Faltings's Theorem. Lang, Serge. Survey of Diophantine geometry. Springer-Verlag. Pp. 101–122. ISBN 3-540-61223-8. Manin, Ju. I.. "Rational points on algebraic curves over function fields".

Izvestiya Akademii Nauk SSSR. Seriya Matematicheskaya. 27: 1395–1440. ISSN 0373-2436. MR 0157971. Mordell, Louis J.. "On the rational solutions of the indeterminate equation of the third and fourth degrees". Proc. Cambridge Philos. Soc. 21: 179–192. Paršin, A. N.. "Quelques conjectures de finitude en géométrie diophantienne". Actes du Congrès International des Mathématiciens. Tome 1. Nice: Gauthier-Villars. Pp. 467–471. MR 0427323. Archived from the original on 2016-09-24. Retrieved 2016-06-11. Parshin, A. N. "Mordell conje

Surface of general type

In algebraic geometry, a surface of general type is an algebraic surface with Kodaira dimension 2. Because of Chow's theorem any compact complex manifold of dimension 2 and with Kodaira dimension 2 will be an algebraic surface, in some sense most surfaces are in this class. Gieseker showed, it remains a difficult problem to describe these schemes explicitly, there are few pairs of Chern numbers for which this has been done. There are some indications that these schemes are in general too complicated to write down explicitly: the known upper bounds for the number of components are large, some components can be non-reduced everywhere, components may have many different dimensions, the few pieces that have been studied explicitly tend to look rather complicated; the study of which pairs of Chern numbers can occur for a surface of general type is known as "geography of Chern numbers" and there is an complete answer to this question. There are several conditions that the Chern numbers of a minimal complex surface of general type must satisfy: c 1 2 + c 2 ≡ 0 c 1 2, c 2 ⩾ 0 c 1 2 ⩽ 3 c 2 5 c 1 2 − c 2 + 36 ⩾ 12 q ⩾ 0 where q is the irregularity of a surface.

Many pairs of integers satisfying these conditions are the Chern numbers for some complex surface of general type. By contrast, for complex surfaces, the only constraint is: c 1 2 + c 2 ≡ 0, this can always be realized; this is only a small selection of the rather large number of examples of surfaces of general type that have been found. Many of the surfaces of general type that have been investigated lie on the edges of the region of possible Chern numbers. In particular Horikawa surfaces lie on or near the "Noether line", many of the surfaces listed below lie on the line c 1 2 + c 2 = 12 χ = 12, the minimum possible value for general type, surfaces on the line 3 c 2 = c 1 2 are all quotients of the unit ball in C2; these surface which are located in the "lower left" boundary in the diagram have been studied in detail. For these surfaces with second Chern class can be any integer from 3 to 11. Surfaces with all these values are known; the first example was found by Mumford using p-adic geometry, there are 50 examples altogether.

They have the same Betti numbers as the projective plane, but are not homeomorphic to it as their fundamental groups are infinite. C2 = 4: Beauville surfaces are named for Arnaud Beauville and have infinite fundamental group. C2 ≥ 4: Burniat surfaces c2 = 10: Campedelli surfaces. Surfaces with the same Hodge numbers are called numerical Campedelli surfaces. C2 = 10: Catanese surfaces are connected. C2 = 11: Godeaux surfaces; the cyclic group of order 5 acts on the Fermat surface of points in P3 satisfying w 5 + x 5 + y 5 + z 5 = 0 by mapping to where ρ is a fifth root of 1. The quotient by this action is the original Godeaux surface. Other surfaces constructed in a similar way with the same Hodge numbers are sometimes called Godeaux surfaces. Surfaces with the same Hodge numbers are called numerical Godeaux surfaces; the fundamental group is cyclic of order 5. C2 = 11: Barlow surfaces are connected, are the only known examples of connected surfaces of general type with pg = 0. Todorov surfaces give counterexamples to the conclusion of the Torelli theorem Castelnuovo surfaces: Another extremal case, Castelnuovo proved that if the canonical bundle is ample for a surface of general type c