In abstract algebra, a monomial ideal is an ideal generated by monomials in a multivariate polynomial ring over a field.
Definitions and properties Let K {\displaystyle \mathbb {K} } be a field and R = K [ x ] {\displaystyle R=\mathbb {K} [x]} be the polynomial ring over K {\displaystyle \mathbb {K} } with n indeterminates x = x 1 , x 2 , … , x n {\displaystyle x=x_{1},x_{2},\dotsc ,x_{n}} . A monomial in R {\displaystyle R} is a product x α = x 1 α 1 x 2 α 2 ⋯ x n α n {\displaystyle x^{\alpha }=x_{1}^{\alpha _{1}}x_{2}^{\alpha _{2}}\cdots x_{n}^{\alpha _{n}}} for an n-tuple α = ( α 1 , α 2 , … , α n ) ∈ N n {\displaystyle \alpha =(\alpha _{1},\alpha _{2},\dotsc ,\alpha _{n})\in \mathbb {N} ^{n}} of nonnegative integers. The following three conditions are equivalent for an ideal I ⊆ R {\displaystyle I\subseteq R} :
I {\displaystyle I} is generated by monomials, If f = ∑ α ∈ N n c α x α ∈ I {\textstyle f=\sum _{\alpha \in \mathbb {N} ^{n}}c_{\alpha }x^{\alpha }\in I} , then x α ∈ I {\displaystyle x^{\alpha }\in I} , provided that c α {\displaystyle c_{\alpha }} is nonzero.
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