Subset Product/Examples/Subset Product with Empty Set
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Example of Subset Product
Let $\struct {S, \circ}$ be an algebraic structure.
Let $A, B \in \powerset S$.
If $A = \O$ or $B = \O$, then $A \circ B = \O$.
Proof
We show the contrapositive:
Suppose that $A \circ B \ne \O$.
Then:
- $\exists x: x \in A \circ B$
From definition of subset product:
- $A \circ B = \set {a \circ b: a \in A, b \in B}$
Therefore:
- $\exists a \in A, b \in B: x = a \circ b$
In particular, both $A$ and $B$ are non-empty.
By De Morgan's Laws (Logic)/Conjunction of Negations, it is not the case that $A = \O$ or $B = \O$.
Hence the result by Proof by Contraposition.
$\blacksquare$
Sources
- 1978: Thomas A. Whitelaw: An Introduction to Abstract Algebra ... (previous) ... (next): $\S 41$: Multiplication of subsets of a group