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Morphism Of Schemes
In algebraic geometry, a morphism of schemes generalizes a morphism of algebraic varieties just as a scheme generalizes an algebraic variety. It is, by definition, a morphism in the category of schemes. A morphism of algebraic stacks generalizes a morphism of schemes. Definition By definition, a morphism of schemes is just a morphism of locally ringed spaces. Isomorphisms are defined accordingly. A scheme, by definition, has open affine charts and thus a morphism of schemes can also be described in terms of such charts (compare the definition of morphism of varieties). Let ƒ:''X''→''Y'' be a morphism of schemes. If ''x'' is a point of ''X'', since ƒ is continuous, there are open affine subsets ''U'' = Spec ''A'' of ''X'' containing ''x'' and ''V'' = Spec ''B'' of ''Y'' such that ƒ(''U'') ⊆ ''V''. Then ƒ: ''U'' → ''V'' is a morphism of affine schemes and thus is induced by some ring homomorphism ''B'' → ''A'' (cf. #Affine case.) In fact, one can use this des ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Morphism Of Algebraic Varieties
In algebraic geometry, a morphism between algebraic varieties is a function between the varieties that is given locally by polynomials. It is also called a regular map. A morphism from an algebraic variety to the affine line is also called a regular function. A regular map whose inverse is also regular is called biregular, and the biregular maps are the isomorphisms of algebraic varieties. Because regular and biregular are very restrictive conditions – there are no non-constant regular functions on projective varieties – the concepts of rational and birational maps are widely used as well; they are partial functions that are defined locally by rational fractions instead of polynomials. An algebraic variety has naturally the structure of a locally ringed space; a morphism between algebraic varieties is precisely a morphism of the underlying locally ringed spaces. Definition If ''X'' and ''Y'' are closed subvarieties of \mathbb^n and \mathbb^m (so they are affine varieties ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Zariski Tangent Space
In algebraic geometry, the Zariski tangent space is a construction that defines a tangent space at a point ''P'' on an algebraic variety ''V'' (and more generally). It does not use differential calculus, being based directly on abstract algebra, and in the most concrete cases just the theory of a system of linear equations. Motivation For example, suppose ''C'' is a plane curve defined by a polynomial equation :''F''(''X,Y'') ''= 0'' and take ''P'' to be the origin (0,0). Erasing terms of higher order than 1 would produce a 'linearised' equation reading :''L''(''X,Y'') ''= 0'' in which all terms ''XaYb'' have been discarded if ''a + b > 1''. We have two cases: ''L'' may be 0, or it may be the equation of a line. In the first case the (Zariski) tangent space to ''C'' at (0,0) is the whole plane, considered as a two-dimensional affine space. In the second case, the tangent space is that line, considered as affine space. (The question of the origin comes up, when we take ''P'' a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gluing Schemes
In algebraic geometry, a new scheme (e.g. an algebraic variety) can be obtained by gluing existing schemes through gluing maps. Statement Suppose there is a (possibly infinite) family of schemes \_ and for pairs i, j, there are open subsets U_ and isomorphisms \varphi_ : U_ \overset\to U_. Now, if the isomorphisms are compatible in the sense: for each i, j, k, # \varphi_ = \varphi_^, # \varphi_(U_ \cap U_) = U_ \cap U_, # \varphi_ \circ \varphi_ = \varphi_ on U_ \cap U_, then there exists a scheme ''X'', together with the morphisms \psi_i : X_i \to X such that # \psi_i is an isomorphism onto an open subset of ''X'', # X = \cup_i \psi_i(X_i), # \psi_i(U_) = \psi_i(X_i) \cap \psi_j(X_j), # \psi_i = \psi_j \circ \varphi_ on U_. Examples Projective line Let X = \operatorname(k \simeq \mathbb^1, Y = \operatorname(k \simeq \mathbb^1 be two copies of the affine line over a field ''k''. Let X_t = \ = \operatorname(k , t^ be the complement of the origin and Y_u = \ defined simila ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Analytic Space
An analytic space is a generalization of an analytic manifold that allows singularities. An analytic space is a space that is locally the same as an analytic variety. They are prominent in the study of several complex variables, but they also appear in other contexts. Definition Fix a field ''k'' with a valuation. Assume that the field is complete and not discrete with respect to this valuation. For example, this includes R and C with respect to their usual absolute values, as well as fields of Puiseux series with respect to their natural valuations. Let ''U'' be an open subset of ''k''''n'', and let ''f''1, ..., ''f''''k'' be a collection of analytic functions on ''U''. Denote by ''Z'' the common vanishing locus of ''f''1, ..., ''f''''k'', that is, let ''Z'' = . ''Z'' is an analytic variety. Suppose that the structure sheaf of ''U'' is \mathcal_U. Then ''Z'' has a structure sheaf \mathcal_Z = \mathcal_U / \mathcal_Z, where \mathcal_Z is the ideal sheaf generated by ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hausdorff Topological Space
In topology and related branches of mathematics, a Hausdorff space ( , ), T2 space or separated space, is a topological space where distinct points have disjoint neighbourhoods. Of the many separation axioms that can be imposed on a topological space, the "Hausdorff condition" (T2) is the most frequently used and discussed. It implies the uniqueness of limits of sequences, nets, and filters. Hausdorff spaces are named after Felix Hausdorff, one of the founders of topology. Hausdorff's original definition of a topological space (in 1914) included the Hausdorff condition as an axiom. Definitions Points x and y in a topological space X can be '' separated by neighbourhoods'' if there exists a neighbourhood U of x and a neighbourhood V of y such that U and V are disjoint (U\cap V=\varnothing). X is a Hausdorff space if any two distinct points in X are separated by neighbourhoods. This condition is the third separation axiom (after T0 and T1), which is why Hausdorff spa ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Quasi-affine Variety
In mathematics, a quasi-projective variety in algebraic geometry is a locally closed subset of a projective variety, i.e., the intersection inside some projective space of a Zariski-open and a Zariski-closed subset. A similar definition is used in scheme theory, where a ''quasi-projective scheme'' is a locally closed subscheme of some projective space. Relationship to affine varieties An affine space is a Zariski-open subset of a projective space, and since any closed affine subset U can be expressed as an intersection of the projective completion \bar and the affine space embedded in the projective space, this implies that any affine variety is quasiprojective. There are locally closed subsets of projective space that are not affine, so that quasi-projective is more general than affine. Taking the complement of a single point in projective space of dimension at least 2 gives a non-affine quasi-projective variety. This is also an example of a quasi-projective variety that is neithe ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Closed Immersion
In algebraic geometry, a closed immersion of schemes is a morphism of schemes f: Z \to X that identifies ''Z'' as a closed subset of ''X'' such that locally, regular functions on ''Z'' can be extended to ''X''. The latter condition can be formalized by saying that f^\#:\mathcal_X\rightarrow f_\ast\mathcal_Z is surjective. An example is the inclusion map \operatorname(R/I) \to \operatorname(R) induced by the canonical map R \to R/I. Other characterizations The following are equivalent: #f: Z \to X is a closed immersion. #For every open affine U = \operatorname(R) \subset X, there exists an ideal I \subset R such that f^(U) = \operatorname(R/I) as schemes over ''U''. #There exists an open affine covering X = \bigcup U_j, U_j = \operatorname R_j and for each ''j'' there exists an ideal I_j \subset R_j such that f^(U_j) = \operatorname (R_j / I_j) as schemes over U_j. #There is a quasi-coherent sheaf of ideals \mathcal on ''X'' such that f_\ast\mathcal_Z\cong \mathcal_X/\mathcal an ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Finitely Generated Algebra
In mathematics, a finitely generated algebra (also called an algebra of finite type) is a commutative associative algebra ''A'' over a field ''K'' where there exists a finite set of elements a_1,\dots,a_n of ''A'' such that every element of ''A'' can be expressed as a polynomial in a_1,\dots,a_n, with coefficients in ''K''. Equivalently, there exist elements a_1,\dots,a_n\in A such that the evaluation homomorphism at =(a_1,\dots,a_n) :\phi_\colon K _1,\dots,X_ntwoheadrightarrow A is surjective; thus, by applying the first isomorphism theorem, A \simeq K _1,\dots,X_n(\phi_). Conversely, A:= K _1,\dots,X_nI for any ideal I\subseteq K _1,\dots,X_n/math> is a K-algebra of finite type, indeed any element of A is a polynomial in the cosets a_i:=X_i+I, i=1,\dots,n with coefficients in K. Therefore, we obtain the following characterisation of finitely generated K-algebras :A is a finitely generated K-algebra if and only if it is isomorphic as a K-algebra to a quotient ring of the type ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Morphism Of Finite Type
In commutative algebra, given a homomorphism A\to B of commutative rings, B is called an A-algebra of finite type if B can be finitely generated as an A-algebra. It is much stronger for B to be a finite A-algebra, which means that B is finitely generated as an A- module. For example, for any commutative ring A and natural number n, the polynomial ring A _1,\dots,x_n/math> is an A-algebra of finite type, but it is not a finite A-algebra unless A = 0 or n = 0. Another example of a finite-type homomorphism that is not finite is \mathbb \to \mathbb x,y]/(y^2 - x^3 - t). The analogous notion in terms of scheme (mathematics), schemes is that a morphism f:X\to Y of schemes is of finite type if Y has a covering by affine open subschemes V_i=\operatorname(A_i) such that f^(V_i) has a finite covering by affine open subschemes U_=\operatorname(B_) of X with B_ an A_i-algebra of finite type. One also says that X is of finite type over Y. For example, for any natural number n and field k, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Diagonal Embedding
In algebraic geometry, given a morphism of schemes p: X \to S, the diagonal morphism :\delta: X \to X \times_S X is a morphism determined by the universal property of the fiber product X \times_S X of ''p'' and ''p'' applied to the identity 1_X : X \to X and the identity 1_X. It is a special case of a graph morphism: given a morphism f: X \to Y over ''S'', the graph morphism of it is X \to X \times_S Y induced by f and the identity 1_X. The diagonal embedding is the graph morphism of 1_X. By definition, ''X'' is a separated scheme over ''S'' (p: X \to S is a separated morphism) if the diagonal morphism is a closed immersion. Also, a morphism p: X \to S locally of finite presentation is an unramified morphism if and only if the diagonal embedding is an open immersion. Explanation As an example, consider an algebraic variety over an algebraically closed field ''k'' and p: X \to \operatorname(k) the structure map. Then, identifying ''X'' with the set of its ''k''-rational points, X ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Fiber Product Of Schemes
In mathematics, specifically in algebraic geometry, the fiber product of schemes is a fundamental construction. It has many interpretations and special cases. For example, the fiber product describes how an algebraic variety over one field determines a variety over a bigger field, or the pullback of a family of varieties, or a fiber of a family of varieties. Base change is a closely related notion. Definition The category of schemes is a broad setting for algebraic geometry. A fruitful philosophy (known as Grothendieck's relative point of view) is that much of algebraic geometry should be developed for a morphism of schemes ''X'' → ''Y'' (called a scheme ''X'' over ''Y''), rather than for a single scheme ''X''. For example, rather than simply studying algebraic curves, one can study families of curves over any base scheme ''Y''. Indeed, the two approaches enrich each other. In particular, a scheme over a commutative ring ''R'' means a scheme ''X'' together with a morphism '' ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Nilpotent
In mathematics, an element x of a ring (mathematics), ring R is called nilpotent if there exists some positive integer n, called the index (or sometimes the degree), such that x^n=0. The term, along with its sister Idempotent (ring theory), idempotent, was introduced by Benjamin Peirce in the context of his work on the classification of algebras. Examples *This definition can be applied in particular to square matrix, square matrices. The matrix :: A = \begin 0 & 1 & 0\\ 0 & 0 & 1\\ 0 & 0 & 0 \end :is nilpotent because A^3=0. See nilpotent matrix for more. * In the factor ring \Z/9\Z, the equivalence class of 3 is nilpotent because 32 is Congruence relation, congruent to 0 Modular arithmetic, modulo 9. * Assume that two elements a and b in a ring R satisfy ab=0. Then the element c=ba is nilpotent as \beginc^2&=(ba)^2\\ &=b(ab)a\\ &=0.\\ \end An example with matrices (for ''a'', ''b''):A = \begin 0 & 1\\ 0 & 1 \end, \;\; B =\begin 0 & 1\\ ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
