Denial of Universality/Examples/x less than or equal to 3/Examples/0 to 3
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Example of Denial of Universality: $\forall x \in S: x \le 3$
Let $P$ be the statement:
- $\forall x \in S: x \le 3$
and $\lnot P$ its negation:
- $\exists x \in S: x > 3$
Let $S = \closedint 0 3$ where $\closedint \cdot \cdot$ denotes a closed real interval.
Then we have that:
- $P$ is true
and consequently:
- $\lnot P$ is false
Proof
By definition of closed real interval:
- $\forall x \in S: 0 \le x \le 3$
and so:
- $\forall x \in S: x \le 3$
$\blacksquare$
Sources
- 1978: Thomas A. Whitelaw: An Introduction to Abstract Algebra ... (previous) ... (next): Chapter $1$: Sets and Logic: Exercise $1 \ \text {(i)} \, \text {(b)}$