Ideal induces Congruence Relation on Ring
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Theorem
Let $\struct {R, +, \circ}$ be a ring.
Let $J$ be an ideal of $R$
Then $J$ induces a congruence relation $\EE_J$ on $R$ such that $\struct {R / J, +, \circ}$ is a quotient ring.
Proof
From Ideal is Additive Normal Subgroup, we have that $\struct {J, +}$ is a normal subgroup of $\struct {R, +}$.
Let $x \mathop {\EE_J} y$ denote that $x$ and $y$ are in the same coset, that is:
- $x \mathop {\EE_J} y \iff x + N = y + N$
From Congruence Modulo Normal Subgroup is Congruence Relation, $\EE_J$ is a congruence relation for $+$.
Now let $x \mathop {\EE_J} x', y \mathop {\EE_J} y'$.
By definition of congruence modulo $J$:
- $x + \paren {-x'} \in J$
- $y + \paren {-y'} \in J$
Then:
- $x \circ y + \paren {-x' \circ y'} = \paren {x + \paren {-x'} } \circ y + x' \circ \paren {y + \paren {-y'} } \in J$
demonstrating that $\EE_J$ is a congruence relation for $\circ$.
Hence the result by definition of quotient ring.
$\blacksquare$
Also see
Sources
- 1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $\text {IV}$: Rings and Fields: $22$. New Rings from Old: Theorems $22.3$