Suppose that $A_i = {1, 2, 3, \ldots, i}$ for $i = 1, 2, 3, \ldots$
Then find $\displaystyle \bigcup_{i=1}^{\infty} A_i = ?$
Here $Z$ denotes the set of integers.
$,\text{Find } \displaystyle \bigcup_{i=1}^{\infty} A_i \text{ and } \bigcap_{i=1}^{\infty} A_i,\ \text{for every positive integer } i \text{ where } A_i={-i,i}.$
(Here $\mathbb Z$ denotes the set of integers.)
Which of the following relations are functions?
(i) ${(1,(a,b)),\ (2,(b,c)),\ (3,(c,a)),\ (4,(a,b))}$
(ii) ${(1,(a,b)),\ (2,(b,a)),\ (3,(c,a)),\ (1,(a,c))}$
(iii) ${(1,(a,b)),\ (2,(a,b)),\ (3,(a,b))}$
(iv) ${(1,(a,b)),\ (2,(b,c)),\ (1,(c,a))}$
A relation $R$ is a function iff every first component occurs exactly once.
(i) first components $1,2,3,4$ appear once each $\Rightarrow$ function.
(ii) $1$ appears twice $\Rightarrow$ not a function.
(iii) $1,2,3$ appear once each $\Rightarrow$ function.
(iv) $1$ appears twice $\Rightarrow$ not a function.
Let total mathematics students be $M$, chemistry students be $C$, and both be $30$.
\[
0.1M=30 \Rightarrow M=300,\qquad 0.12C=30 \Rightarrow C=250.
\]
At least one $= M+C-\text{both}=300+250-30=520$.
R includes $(1,1), (2,2), (3,3)$ → Reflexive.
Check symmetry: $(1,2)$ exists but $(2,1)$ does not → Not symmetric.
Check transitivity: $(1,2)$ and $(2,2)$ imply $(1,2)$ already → transitive holds.
Hence, relation is reflexive and transitive but not symmetric.
For $a, b \in \mathbb{R}$ define $a = b$ to mean that $|x| = |y|$.
If $[x]$ is an equivalence relation in $R$, then the equivalence relation for $[17]$ is...
Reflexive — yes (all $(a,a)$ are present).
Symmetric — no, since $(1,2)\in R$ but $(2,1)\notin R$.
Transitive — yes, because $(1,2)$ and $(2,3)$ imply $(1,3)$ (which exists).
For reflexivity: $xRx$ means $x-x+\sqrt{2}=\sqrt{2}$ (irrational) ⇒ true.
For symmetry: $xRy⇒x-y+\sqrt{2}$ irrational, but $y-x+\sqrt{2}=-(x-y)+\sqrt{2}$ may be rational. Not always true ⇒ not symmetric.
For transitivity: fails similarly.
$(A \cup B)'$ means elements not in $A$ or $B$.
Hence, $(A \cup B)' \cap B$ contains elements that are both in $B$ and not in $B$, i.e., none.
So result is $\phi$.
Symmetric: $(1,2)\in R$ but $(2,1)\notin R\Rightarrow$ not symmetric.
Transitive: check the only nontrivial chain: $(1,2)$ and $(2,3)\Rightarrow (1,3)$, which is in $R$.
Pairs with $(x,x)$ keep the second pair; $(1,3)$ can only compose with $(3,3)$ giving $(1,3)$; $(2,3)$ with $(3,3)$ gives $(2,3)$. All required compositions are in $R$.
$\boxed{\text{$R$ is reflexive and transitive, but not symmetric.}}$
Not reflexive (missing $(1,1),(2,2),(3,3)$). Not symmetric (since $(2,1)\notin R$). Transitive holds vacuously because there is no pair starting at $2$ to trigger $(1,2)$∘$(2,\cdot)$.
A, C, and D are bijections (permutations), hence invertible. In B, both $2$ and $3$ map to $1$ (not one-to-one), so not bijective $\Rightarrow$ no inverse.