Right Operation is Distributive over Idempotent Operation
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Theorem
Let $\struct {S, \circ, \rightarrow}$ be an algebraic structure where:
- $\rightarrow$ is the right operation
- $\circ$ is any arbitrary binary operation.
Then:
- $\rightarrow$ is distributive over $\circ$
- $\circ$ is idempotent.
Proof
From Right Operation is Left Distributive over All Operations:
- $\forall a, b, c \in S: a \rightarrow \paren {b \circ c} = \paren {a \rightarrow b} \circ \paren {a \rightarrow c}$
for all binary operations $\circ$.
It remains to show that $\rightarrow$ is right distributive over $\circ$ if and only if $\circ$ is idempotent.
Necessary Condition
Let $\circ$ be idempotent.
Then:
\(\ds \paren {a \circ b} \rightarrow c\) | \(=\) | \(\ds c\) | Definition of Right Operation | |||||||||||
\(\ds \) | \(=\) | \(\ds c \circ c\) | Definition of Idempotent Operation | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {a \rightarrow c} \circ \paren {b \rightarrow c}\) | Definition of Right Operation |
Thus $\rightarrow$ is right distributive over $\circ$.
$\Box$
Sufficient Condition
Let $\rightarrow$ be right distributive over $\circ$.
Let $c \in S$ be arbitrary.
Then:
\(\ds c\) | \(=\) | \(\ds \paren {a \circ b} \rightarrow c\) | Definition of Right Operation | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {a \rightarrow c} \circ \paren {b \rightarrow c}\) | Definition of Right Distributive Operation | |||||||||||
\(\ds \) | \(=\) | \(\ds c \circ c\) | Definition of Right Operation |
Hence $\circ$ is idempotent.
$\blacksquare$
Sources
- 1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $\text {III}$: The Natural Numbers: $\S 16$: The Natural Numbers: Exercise $16.23 \ \text{(b)}$