Finitely generated algebra

• The polynomial algebra K[x₁,...,x_n ] is finitely generated. The polynomial algebra in countably infinitely many generators is infinitely generated.
• The field E = K(t) of rational functions in one variable over an infinite field K is not a finitely generated algebra over K. On the other hand, E is generated over K by a single element, t, as a field.
• If E/F is a finite field extension then it follows from the definitions that E is a finitely generated algebra over F.
• Conversely, if E/F is a field extension and E is a finitely generated algebra over F then the field extension is finite. This is called Zariski's lemma. See also integral extension.
• If G is a finitely generated group then the group algebra KG is a finitely generated algebra over K.

• A homomorphic image of a finitely generated algebra is itself finitely generated. However, a similar property for subalgebras does not hold in general.
• Hilbert's basis theorem: if A is a finitely generated commutative algebra over a Noetherian ring then every ideal of A is finitely generated, or equivalently, A is a Noetherian ring.

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),

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

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.

• Finitely generated module
• Finitely generated field extension
• Artin–Tate lemma
• Finite algebra
• Morphism of finite type