Negate Uniqueness: Check Correctness

[Jeff Ford]

I think I've got this one, I'd just like someone to check my work
Negate the statement (\exists! x \in S) P(x)

Since (\exists ! x \in S) P(x) \Longleftrightarrow \{(\exists x \in S) (P(x) \} \wedge \{(\forall x,y \in S) [P(x) \wedge P(y) \longrightarrow x = y \}

The negation would be \sim (\exists ! x \in S) P(x) \Longleftrightarrow \{(\forall x \in S) \sim P(x)\} \wedge \{\exists x,y \in S) \sim [P(x) \wedge P(y) \longrightarrow x = y \}

Does this look correct?

 

[honestrosewater]

I think I've got this one, I'd just like someone to check my work
Negate the statement (\exists! x \in S) P(x)
Since (\exists ! x \in S) P(x) \Longleftrightarrow \{(\exists x \in S) (P(x) \} \wedge \{(\forall x,y \in S) [P(x) \wedge P(y) \longrightarrow x = y \}
The negation would be \sim (\exists ! x \in S) P(x) \Longleftrightarrow \{(\forall x \in S) \sim P(x)\} \wedge \{\exists x,y \in S) \sim [P(x) \wedge P(y) \longrightarrow x = y \}
Does this look correct?

Flip something upside down.
\neg(P \wedge Q) \Leftrightarrow (\neg P \vee \neg Q)

 

[honestrosewater]

Sorry, my comment may not have been clear, and I read that you're teaching yourself (), so I'll try to explain just to be safe. (I use grouping symbols a bit differently; I think it makes more sense this way.)

(\exists ! x \in S)(Px)

has two parts, existence and uniqueness.

\mbox{Existence: } (\exists x \in S)(Px)
\mbox{Uniqueness: } (\forall x, y \in S)((Px \wedge Py) \rightarrow (x = y))

Call the existence statement E and the uniqueness statement U. The right-hand side of your definition of (\exists ! x \in S)(Px) can then be stated as

(E \wedge U)

And its negation is

(\neg[E \wedge U]) \Leftrightarrow (\neg E \vee \neg U)

So just plug back in the definitions of E and U to get the right-hand side of \neg[(\exists ! x \in S)(Px)].

(\neg[(\exists x \in S)(Px)]) \ \vee \ (\neg[(\forall x, y \in S)((Px \wedge Py) \rightarrow (x = y))])

which you can further simplify by "distributing" the negations. Here are some handy http://people.cornell.edu/pages/ps92/414/LogicalOpLogicQuantifiers.pdf (PDF); see page 4 for a shorter way to write a statement of unique existence.