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# 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 a1,...,an of A such that every element of A can be expressed as a polynomial in a1,...,an, with coefficients in K.

Equivalently, there exist elements ${\displaystyle a_{1},\dots ,a_{n}\in A}$ s.t. the evaluation homomorphism at ${\displaystyle {\bf {a}}=(a_{1},\dots ,a_{n})}$

${\displaystyle \phi _{\bf {a}}\colon K[X_{1},\dots ,X_{n}]\twoheadrightarrow A}$

is surjective; thus, by applying the first isomorphism theorem ${\displaystyle A\simeq K[X_{1},\dots ,X_{n}]/{\rm {ker}}(\phi _{\bf {a}})}$.

Conversely, ${\displaystyle A:=K[X_{1},\dots ,X_{n}]/I}$ for any ideal ${\displaystyle I\subset K[X_{1},\dots ,X_{n}]}$ is a ${\displaystyle K}$-algebra of finite type, indeed any element of ${\displaystyle A}$ is a polynomial in the cosets ${\displaystyle a_{i}:=X_{i}+I,i=1,\dots ,n}$ with coefficients in ${\displaystyle K}$. Therefore, we obtain the following characterisation of finitely generated ${\displaystyle K}$-algebras[1]

${\displaystyle A}$ is a finitely generated ${\displaystyle K}$-algebra if and only if it is isomorphic to a quotient ring of the type ${\displaystyle K[X_{1},\dots ,X_{n}]/I}$ by an ideal ${\displaystyle I\subset K[X_{1},\dots ,X_{n}]}$.

If it is necessary to emphasize the field K then the algebra is said to be finitely generated over K . Algebras that are not finitely generated are called infinitely generated.

## Relation with affine varieties

Finitely generated reduced commutative algebras are basic objects of consideration in modern algebraic geometry, where they correspond to affine algebraic varieties; for this reason, these algebras are also referred to as (commutative) affine algebras. More precisely, given an affine algebraic set ${\displaystyle V\subset \mathbb {A} ^{n}}$ we can associate a finitely generated ${\displaystyle K}$-algebra

${\displaystyle \Gamma (V):=K[X_{1},\dots ,X_{n}]/I(V)}$

called the affine coordinate ring of ${\displaystyle V}$; moreover, if ${\displaystyle \phi \colon V\to W}$ is a regular map between the affine algebraic sets ${\displaystyle V\subset \mathbb {A} ^{n}}$ and ${\displaystyle W\subset \mathbb {A} ^{m}}$, we can define a homomorphism of ${\displaystyle K}$-algebras

${\displaystyle \Gamma (\phi )\equiv \phi ^{*}\colon \Gamma (W)\to \Gamma (V),\,\phi ^{*}(f)=f\circ \phi ,}$

then, ${\displaystyle \Gamma }$ is a contravariant functor from the category of affine algebraic sets with regular maps to the category of reduced finitely generated ${\displaystyle K}$-algebras: this functor turns out[2]to be an equivalence of categories

${\displaystyle \Gamma \colon ({\text{affine algebraic sets}})^{\rm {opp}}\to ({\text{reduced finitely generated }}K{\text{-algebras}}),}$

and, restricting to affine varieties (i.e. irreducible affine algebraic sets),

${\displaystyle \Gamma \colon ({\text{affine algebraic varieties}})^{\rm {opp}}\to ({\text{integral finitely generated }}K{\text{-algebras}}).}$

## Finite algebras vs algebras of finite type

We recall that a commutative ${\displaystyle R}$-algebra ${\displaystyle A}$ is a ring homomorphism ${\displaystyle \phi \colon R\to A}$; the ${\displaystyle R}$-module structure of ${\displaystyle A}$ is defined by

${\displaystyle \lambda \cdot a:=\phi (\lambda )a,\quad \lambda \in R,a\in A.}$

An ${\displaystyle R}$-algebra ${\displaystyle A}$ is finite if it is finitely generated as an ${\displaystyle R}$-module, i.e. there is a surjective homomorphism of ${\displaystyle R}$-modules

${\displaystyle R^{\oplus _{n}}\twoheadrightarrow A.}$

Again, there is a characterisation of finite algebras in terms of quotients[3]

An ${\displaystyle R}$-algebra ${\displaystyle A}$ is finite if and only if it is isomorphic to a quotient ${\displaystyle R^{\oplus _{n}}/M}$ by an ${\displaystyle R}$-submodule ${\displaystyle M\subset R}$.

By definition, a finite ${\displaystyle R}$-algebra is of finite type, but the converse is false: the polynomial ring ${\displaystyle R[X]}$ is of finite type but not finite.

Finite algebras and algebras of finite type are related to the notions of finite morphisms and morphisms of finite type.

## References

1. ^ Kemper, Gregor (2009). A Course in Commutative Algebra. Springer. p. 8. ISBN 978-3-642-03545-6.
2. ^ Görtz, Ulrich; Wedhorn, Torsten (2010). Algebraic Geometry I. Schemes With Examples and Exercises. Springer. p. 19. ISBN 978-3-8348-0676-5.
3. ^ Atiyah, Michael Francis; MacDonald, Ian Grant (1994). Introduction to commutative algebra. CRC Press. p. 21. ISBN 9780201407518.