Translation of Integer Interval is Bijection
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Theorem
Let $a, b, c \in \Z$ be integers.
Let $\closedint a b$ denote the integer interval between $a$ and $b$.
Then the mapping $T: \closedint a b \to \closedint {a + c} {b + c}$ defined as:
- $\map T k = k + c$
is a bijection.
Proof
Note that if $k \in \closedint a b$, then indeed $k + c \in \closedint {a + c} {b + c}$.
Injectivity
Let $k, l \in \closedint a b$ with $k + c = l + c$.
By Integer Addition is Cancellable, $k = l$.
Thus $T$ is injective.
$\Box$
Surjectivity
Let $m \in \closedint {a + c} {b + c}$.
Then $m - c \in \closedint a b$.
Then $\map T {m - c} = m - c + c = m$.
Thus $T$ is surjective.
$\blacksquare$