# Equivalence of Definitions of Integral Element of Algebra

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## Theorem

The following definitions of the concept of **Integral Element of Algebra** are equivalent:

Let $A$ be a commutative ring with unity.

Let $f : A \to B$ be a commutative $A$-algebra.

Let $b\in B$.

### Definition 1

The element $b$ is **integral** over $A$ if and only if it is a root of a monic polynomial in $A[x]$.

### Definition 2

The element $b$ is **integral** over $A$ if and only if the generated subalgebra $A[b]$ is a finitely generated module over $A$.

### Definition 3

The element $b$ is **integral** over $A$ if and only if the generated subalgebra $A[b]$ is contained in a subalgebra $C\leq B$ which is a finitely generated module over $A$.

### Definition 4

The element $b$ is **integral** over $A$ if and only if there exists a faithful $A[b]$-module whose restriction of scalars to $A$ is finitely generated.