How come $x \notin N \cup P$ becomes $(x \notin N \text{ and }x \notin P)$ and $x \notin N \cap P$ becomes $(x \notin N \text{ or }x \notin P)$?
116305
Proving DeMorgan's Theorem
125008
Define a bijection between $Z^{X \cup Y}$ and $Z^X \times Z^Y$, where $X, Y \in \mathcal{P}(\Omega) $ and $X \cap Y = \emptyset$
99444
Why do some words have "X" as a substitute?
61899
Prove that the language $\{bin(p) \mid p\ \text{is prime}\}$ is not regular (prime numbers)
49385
Proving $\forall x (A\to B) \to(A \to \forall x B):x\notin \mbox{free}(A)$ in a Hilbert system where it is not an axiom
65379
Deriving $\neg R$ from $\{R↔(R∨(P∧¬P)), R↔¬P, ¬P→(P↔(Q→Q)), P→Q\}$
62208
If $A$ is a non-empty set, then $A \notin A$
85819
A non-polyhedral pair $(X,A)$ with both $X$ and $A$ polyhedral
5840
divides, not divides and cardinalities
60707
Show that if $ \ A \cup B = A$ and $ \ A\cap B = A$ then $ \ A = B$
147047
Does there exist a polynomial $p(x) \in \mathbb C[x]$ such that $p(x) \notin \mathbb R[x]$ and $p(x)p(-x)=p(x^2)$?
16749
Let $A_i= \left \{...,-2,-1,0,1,...,i \right\}$. Find $\bigcup_{i=1}^{n} A_i$ and $\bigcap_{i=1}^{n} A_i$
154363
Prove that $\text{aff}(X) = \text{aff}(\text{closure}(X))$
146891
Why is $\{\emptyset\} \not = \emptyset$?
24708
Why is $\operatorname{Int}(A) \cup \operatorname{Int}(B) \neq \operatorname{Int}(A \cup B)$?
134922
Which notation and why: $\text{P}()$, $\Pr()$, $\text{Prob}()$, or $\mathbb{P}()$
141114
Question about a bijection between $\mathbb R$ and $\mathbb R \cup \{\infty\}$