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In algebraic geometry, an affine variety (or, more generally, an affine algebraic set) is defined as the subset of an affine space that is the set of the common zeros of a set of so-called polynomial functions over the affine space. For defining a polynomial function over the affine space, one has to choose an affine frame.
In projective geometry, affine space means the complement of a hyperplane at infinity in a projective space. Affine space can also be viewed as a vector space whose operations are limited to those linear combinations whose coefficients sum to one, for example 2x − y, x − y + z, (x + y + z)/3, ix + (1 − i)y, etc.
Let X be an affine space over a field k, and V be its associated vector space. An affine transformation is a bijection f from X onto itself that is an affine map; this means that a linear map g from V to V is well defined by the equation () = (); here, as usual, the subtraction of two points denotes the free vector from the second point to the first one, and "well-defined" means that ...
A function : is called affine if it preserves affine combinations. So (+ +) = + + ()for any affine combination + + in A. The space of affine functions A* is a linear space. The dual vector space of A* is naturally isomorphic to an (n+1)-dimensional vector space F(A) which is the free vector space on A modulo the relation that affine combination in A agrees with affine combination in F(A).
Let A 3 be the three-dimensional affine space over C. The set of points (x, x 2, x 3) for x in C is an algebraic variety, and more precisely an algebraic curve that is not contained in any plane. [note 3] It is the twisted cubic shown in the above figure. It may be defined by the equations
Affine space and projective space are smooth schemes over a field k. An example of a smooth hypersurface in projective space P n over k is the Fermat hypersurface x 0 d + ... + x n d = 0, for any positive integer d that is invertible in k. An example of a singular (non-smooth) scheme over a field k is the closed subscheme x 2 = 0 in the affine ...
In geometry, an affine plane is a system of points and lines that satisfy the following axioms: [1]. Any two distinct points lie on a unique line. Given any line and any point not on that line there is a unique line which contains the point and does not meet the given line.
Typical examples of affine planes are Euclidean planes, which are affine planes over the reals equipped with a metric, the Euclidean distance.In other words, an affine plane over the reals is a Euclidean plane in which one has "forgotten" the metric (that is, one does not talk of lengths nor of angle measures).