Cardinality of Set of All Mappings/Examples
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Examples of Cardinality of Set of All Mappings
$2$ Elements to $2$ Elements
Let $X = \set {a, b}$.
Let $Y = \set {u, v}$.
Then the mappings from $X$ to $Y$ are:
\(\text {(1)}: \quad\) | \(\ds \map {f_1} a\) | \(=\) | \(\ds u\) | |||||||||||
\(\ds \map {f_1} b\) | \(=\) | \(\ds v\) |
\(\text {(2)}: \quad\) | \(\ds \map {f_2} a\) | \(=\) | \(\ds u\) | |||||||||||
\(\ds \map {f_2} b\) | \(=\) | \(\ds u\) |
\(\text {(3)}: \quad\) | \(\ds \map {f_3} a\) | \(=\) | \(\ds v\) | |||||||||||
\(\ds \map {f_3} b\) | \(=\) | \(\ds v\) |
\(\text {(4)}: \quad\) | \(\ds \map {f_4} a\) | \(=\) | \(\ds v\) | |||||||||||
\(\ds \map {f_4} b\) | \(=\) | \(\ds u\) |
$f_1$ and $f_4$ are bijections.
$f_2$ and $f_3$ are neither surjections nor injections.
$2$ Elements to $3$ Elements
Let $S = \set {1, 2}$.
Let $T = \set {a, b, c}$.
Then the mappings from $S$ to $T$ in two-row notation are:
- $\dbinom {1 \ 2} {a \ a}, \dbinom {1 \ 2} {a \ b}, \dbinom {1 \ 2} {a \ c}, \dbinom {1 \ 2} {b \ a}, \dbinom {1 \ 2} {b \ b}, \dbinom {1 \ 2} {b \ c}, \dbinom {1 \ 2} {c \ a}, \dbinom {1 \ 2} {c \ b}, \dbinom {1 \ 2} {c \ c}$
a total of $3^2 = 9$.
All but the first, fifth and last are injections.
None are surjections.
$3$ Elements to $2$ Elements
Let $S = \set {1, 2, 3}$.
Let $T = \set {a, b}$.
Then the mappings from $S$ to $T$ in two-row notation are:
- $\dbinom {1 \ 2 \ 3} {a \ a \ a}, \dbinom {1 \ 2 \ 3} {a \ a \ b}, \dbinom {1 \ 2 \ 3} {a \ b \ a}, \dbinom {1 \ 2 \ 3} {a \ b \ b}, \dbinom {1 \ 2 \ 3} {b \ a \ a}, \dbinom {1 \ 2 \ 3} {b \ a \ b}, \dbinom {1 \ 2 \ 3} {b \ b \ a}, \dbinom {1 \ 2 \ 3} {b \ b \ b}$
a total of $2^3 = 8$.
All but the first and last are surjections.
None are injections.
Set of Cardinality $4$ to Itself
Let $S$ be a set whose cardinality is $4$:
- $\card S = 4$
Then there are $256$ mappings from $S$ to itself.