Zero of Cardinal Product is Zero
Jump to navigation
Jump to search
Theorem
Let $\mathbf a$ be a cardinal.
Then:
- $\mathbf 0 \mathbf a = \mathbf 0$
where $\mathbf 0 \mathbf a$ denotes the product of the (cardinal) zero and $\mathbf a$.
That is, $\mathbf 0$ is the zero element of the product operation on cardinals.
Proof
Let $\mathbf a = \card A$ for some set $A$.
From the definition of (cardinal) zero, $\mathbf 0$ is the cardinal associated with the empty set $\O$.
We have by definition of product of cardinals that $\mathbf 0 \mathbf a$ is the cardinal associated with $\O \times A$.
But from Cartesian Product is Empty iff Factor is Empty:
- $\O \times A = \O$
Hence the result.
$\blacksquare$
Sources
- 1975: T.S. Blyth: Set Theory and Abstract Algebra ... (previous) ... (next): $\S 8$: Theorem $8.4: \ (4)$