788 lines
23 KiB
Markdown
788 lines
23 KiB
Markdown
Page 516
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**Exercise Set 8.1**
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1. As in Example 8.1.2, the **congruence modulo $2$** relation $E$ is defined
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from $\mathbb{Z}$ to $\mathbb{Z}$ as follows: For every ordered pair
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$(m, n) \in \mathbb{Z} \times \mathbb{Z}$,
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$$ m E n \Leftrightarrow m - n \text{ is even} $$
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a. Is $0 E 0$? Is $5 E 2$? Is $(6, 6) \in E$? Is $(-1, 7) \in E$?
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_$0 E 0$:_
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Yes, $0 - 0 = 0$, and $0$ is even.
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_$5 E 2$:_
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No, $5 - 2 = 3$, and $3$ is not even.
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_$(6, 6) \in E$:_
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Yes, $6 - 6 = 0$, and $0$ is even.
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_$(-1, 7) \in E$:_
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Yes, $-1 - 7 = -8$, and $-8$ is even.
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b. Prove that for any even integer $n$, $n E 0$.
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**Proof:**
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Suppose $n \in 2\mathbb{Z}$, where $2\mathbb{Z}$ is the set of all even
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integers.
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By the definition for even, this means that $n = 2k$ for some integer $k$.
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By the definition for $E$, $n E 0$ if, and only if $n - 0$ is even.
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By substitution for $E$:
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$$ n - 0 = 2k - 0 $$
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$$ = 2k $$
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By the definition for even, this means that $n - 0$ is even, and therefore
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$n E 0$ is true.
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Q.E.D.
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2. Prove that for all integers $m$ and $n$, $m - n$ is even if, and only if,
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both $m$ and $n$ are even or both $m$ and $n$ are odd.
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_Hint:_ To prove a statement of the form $p \Leftrightarrow (q \vee r)$, you
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need to prove both (1)$p \to (q \vee r)$ and (2) $(q \vee r) \to p$. The easiest
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way to prove $p \to (q \vee r)$ is to prove the logically equivalent statement
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form $(p \wedge \neg q) \to r$. And the easiest way to prove $(q \vee r) \to p$
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is to prove the logically equivalent statement form
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$(q \to p) \wedge (r \to p)$. In this case, suppose $m$ and $n$ are any
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integers, and let $p$ be "$m - n$ is even," let $q$ be "both $m$ and $n$ are
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even," and let $r$ be "both $m$ and $n$ are odd."
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**Proof:**
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Suppose $m$ and $n$ are any integers.
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To prove that for all integers $m$ and $n$, $m - n$ is even if, and only if,
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both $m$ and $n$ are even or both $m$ and $n$ are odd, it must be shown first
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that if $m - n$ is even, then both $m$ and $n$ are even or both $m$ and $n$ are
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odd, then it must be shown second that if both $m$ and $n$ are even or both $m$
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and $n$ are odd, then $m - n$ is even.
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_Proof (first):_
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Suppose $m - n$ is even. To prove that both $m$ and $n$ must be even or both $m$
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and $n$ must be odd, all cases for where $m$ is even or odd and where $n$ is
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even or odd must be considered.
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_Case (both $m$ and $n$ are even):_
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Since both $m$ and $n$ are even, this means that $m = 2k$ and $n = 2p$ for some
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integers $k$ and $p$. Then:
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$$ m - n = 2k - 2p $$
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$$ = 2(k - p) $$
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Now, $k - p$ is an integer by the subtraction of integers. Therefore, by the
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definition of even, $m - n$ is even.
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_Case (both $m$ and $n$ are odd):_
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Since both $m$ and $n$ are odd, this means that $m = 2k + 1$ and $n = 2p + 1$
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for some integers $k$ and $p$. Then:
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$$ m - n = (2k + 1) - (2p + 1) $$
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$$ = 2k + 1 - 2p - 1 $$
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$$ = 2k - 2p $$
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$$ = 2(k - p) $$
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Now, $k - p$ is an integer by the subtraction of integers. Therefore, by the
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definition of even, $m - n$ is even.
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_Case ($m$ is even and $n$ is odd):_
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Since $m$ is even and $n$ is odd, $m = 2k$ and $n = 2p + 1$ for some integers
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$k$ and $p$. Then:
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$$ m - n = 2k - (2p + 1) $$
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$$ = 2k - 2p - 1 $$
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$$ = 2(k - p) - 1 $$
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Now, $k - p$ is an integer by the subtraction of integers. Thus, by the
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definition of odd, $m - n$ is odd, but by the supposition, $m - n$ is even. This
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is a contradiction.
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_Case ($m$ is odd and $n$ is even):_
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Since $m$ is odd and $n$ is even, $m = 2k + 1$ and $n = 2p$ for some integers
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$k$ and $p$. Then:
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$$ m - n = (2k + 1) - 2p $$
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$$ = 2k - 2p + 1 $$
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$$ = 2(k - p) + 1 $$
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Now, $k - p$ is an integer by the subtraction of integers. Thus, by the
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definition of odd, $m - n$ is odd, but by the supposition, $m - n$ is even. This
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is a contradiction.
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_Conclusion:_
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It can be concluded based off of all cases that when both $m$ and $n$ are even
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or both $m$ and $n$ are odd, $m - n$ is even.
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_Proof (second):_
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Suppose both $m$ and $n$ are both even or are both odd.
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In order to prove $m - n$ is even, both cases must be considered.
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_Case (both $m$ and $n$ are even):_
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Since both $m$ and $n$ are even, $m = 2k$ and $n = 2p$ for some integers $k$ and
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$p$. Then:
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$$ m - n = 2k - 2p $$
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$$ = 2(k - p) $$
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Now, $k - p$ is an integer by the subtraction of integers. Therefore, by the
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definition of even, $m - n$ is even.
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_Case (both $m$ and $n$ are odd):_
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Since both $m$ and $n$ are odd, $m = 2k + 1$ and $n = 2p + 1$ for some integers
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$k$ and $p$. Then:
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$$ m - n = (2k + 1) - (2p + 1) $$
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$$ = 2k + 1 - 2p - 1 $$
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$$ = 2k - 2p $$
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$$ = 2(k - p) $$
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Now, $k - p$ is an integer by the subtraction of integers. Therefore, by the
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definition of even, $m - n$ is even.
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_Conclusion:_
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In both cases, $m - n$ is even. Therefore it can be concluded that if both $m$
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and $n$ are even or if both $m$ and $n$ are odd, then $m - n$ is even.
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3. The **congruence modulo $3$** relation, $T$, is defined from $\mathbb{Z}$ to
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$\mathbb{Z}$ as follows: For all integers $m$ and $n$,
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$$ m T n \Leftrightarrow 3 | (m - n) $$
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a. Is $10 T 1$? Is $1 T 10$? Is $(2, 2) \in T$? Is $(8, 1) \in T$?
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_$10 T 1$:_
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Yes, since $3 | (10 - 1) = 3 | 9 = 3$
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_$1 T 10$:_
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Yes, since $3 | (1 - 10) = 3 | -9 = -3$
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_$(2, 2) \in T$:_
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Yes, since $3 | (2 - 2) = 3 | 0 = 0$
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_$(8, 1) \in T$:_
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No, since $3 | (8 - 1) = 3 \cancel{|} 7$.
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b. List five integers $n$ such that $n T 0$.
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$3$; $6$, $9$, $12$, $15$
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c. List five integers $n$ such that $n T 1$.
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$4$; $7$, $10$, $13$, $16$
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d. List five integers $n$ such that $n T 2$.
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$$ 3 | (n - 2) $$
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$5$, $8$, $11$, $14$, $17$
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e. Make and prove a conjecture about which integers are related by $T$ to $0$,
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which integers are related to $T$ to $1$, and which integers are related to $T$
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to $2$.
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_Hint:_ All integers of the form $3k + 1$, for some integer $k$, are related by
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$T$ to $1$.
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**Conjecture:**
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All integers of the form $3k$, for some integer $k$, are related by $T$ to $0$.
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All integers of the form $3p + 1$, for some integer $p$, are related by $T$ to
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$1$.
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All integers of the form $3m + 2$, for some integer $m$, are related to $T$ by
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$2$.
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4. Define a relation $P$ on $\mathbb{Z}$ as follows: For every ordered pair
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$(m, n) \in \mathbb{Z} \times \mathbb{Z}$,
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$$ m P n \Leftrightarrow m \text{ and } n \text{ have a common prime factor} $$
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a. Is $15 P 25$?
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Yes, because both $15$ and $25$ are divisible by $5$, which is a prime factor.
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b. Is $22 P 27$?
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No, because $22$ and $27$ have no common divisors.
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c. Is $0 P 5$?
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Yes, because both $0$ and $5$ are divisible by $5$, which is a prime factor.
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d. Is $8 P 8$?
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Yes, because both $8$ and $8$ are divisible by $2$, which is a prime factor.
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5. Let $X = \{a, b, c\}$. Recall that $\mathscr{P}(X)$ is the power set of $X$.
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Define a relation $\mathbf{S}$ on $\mathscr{P}(X)$ as follows: For all sets
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$A$ and $B$ in $\mathscr{P}(X)$,
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$$ A \mathbf{S}B \Leftrightarrow A \text{ has the same number of elements as } B $$
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a. Is $\{a, b\} \mathbf{S} \{b, c\}$?
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Yes, since both $\{a, b\}$ and $\{b, c}$ have the same number of elements,
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namely $2$ elements.
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b. Is $\{a\} \mathbf{S} \{a, b\}$?
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No, since $\{a\}$ has $1$ element and $\{a, b\}$ has $2$ elements, and
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$1 \neq 2$.
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c. Is $\{c\} \mathbf{S} \{b\}$?
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Yes, since both $\{c\}$ and $\{b\}$ have the same number of elements, namely $1$
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element.
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6. Let $X = \{a, b, c\}$. Recall that $\mathscr{P}(X)$ as follows: For all sets
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$A$ and $B$ in $\mathscr{P}(X)$,
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$$ A \mathbf{J} B \Leftrightarrow A \cap B \neq \emptyset $$
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a. Is $\{a\} \mathbf{J} \{c\}$?
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No, since $\{a\} \cap \{\c} = \emptyset$.
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b. Is $\{a, b\} \mathbf{J} \{b, c\}$?
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Yes, since $\{a, b\} \cap \{b, c\} = \{b\} \neq \emptyset$.
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c. Is $\{a, b} \mathbf{J} \{a, b, c\}$?
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Yes, since $\{a, b\} \cap \{a, b, c\} = \{a, b\} \neq \emptyset$.
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7. Define a relation $R$ on $\mathbb{Z}$ as follows: For all integers $m$ and
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$n$,
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$$ m R n \Leftrightarrow 5 | (m^2 - n^2) $$
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a. Is $1 R (-9)$?
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$$ 5 | ((1)^2 - (-9)^2) $$
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$$ 5 | (1 - 81) $$
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$$ 5 | (-80) = -16 $$
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Yes.
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b. Is $2 R 13$?
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$$ 5 | ((2)^2 - (13)^2) $$
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$$ 5 | (4 - 169) $$
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$$ 5 | (-165) = -33 $$
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Yes.
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c. Is $2 R (-8)$?
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$$ 5 | ((2)^2 - (-8)^2) $$
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$$ 5 | (4 - (64)) $$
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$$ 5 | (-60) = -12 $$
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Yes.
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d. Is $(-8) R 2$?
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$$ 5 | (64 - 4) $$
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$$ 5 | 60 = 12 $$
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Yes.
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8. Let $A$ be the set of all strings of _a_'s and _b_'s of length $4$. Define a
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relation $R$ on $A$ as follows: For every $s, t \in A$,
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$$ s R t \Leftrightarrow s \text{ has the same first two characters as } t $$
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a. Is _abaa_ $R$ _abba_?
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Yes, since _ab_ is the same first two characters of both _abaa_ and _abba_.
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b. Is _aabb_ $R$ _bbaa_?
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No, since _aa_ is the first two characters of _aabb_ and _bb_ is the first same
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two characters as _bbaa_, it can be concluded that _aabb_ and _bbaa_ do not have
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the same first two characters.
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c. Is _aaaa_ $R$ _aaab_?
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Yes, since _aa_ is the same first two characters of both _aaaa_ and _aaab_.
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d. Is _baaa_ $R$ _abaa_?
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No, since _ba_ and _ab_ are the first two characters of _baaa_ and _abaa_
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respectively.
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9. Let $A$ be the set of all strings of 0's, 1's, and 2's of length $4$. Define
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a relation $R$ on $A$ as follows: For every $s, t \in A$,
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$$ s R t \Leftrightarrow \text{ the same of the characters in } s \text{ equals the sum of the characters in } t $$
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a. Is 0121 $R$ 2200?
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$$ 0 + 1 + 2 + 1 = 4 = 2 + 2 + 0 + 0 $$
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Yes.
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b. Is 1011 $R$ 2101?
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$$ 1 + 0 + 1 + 1 = 3 = \neq 4 = 2 + 1 + 0 + 1 $$
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No.
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c. Is 2212 $R$ 2121?
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$$ 2 + 2 + 1 + 2 = 7 \neq 6 = 2 + 1 + 2 + 1 $$
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No.
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d. Is 1220 $R$ 2111?
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$$ 1 + 2 + 2 + 0 = 5 = 2 + 1 + 1 + 1 $$
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Yes.
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10. Let $A = \{3, 4, 5\}$ and $B = \{4, 5, 6\}$ and let $R$ be the "less than"
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relation. That is, for every ordered pair $(x, y) \in A \times B$,
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$$ x R y \Leftrightarrow x < y $$
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State explicitly which ordered pairs are in $R$ and $R^{-1}$.
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$$ R = \{(3, 4), (3, 5), (3, 6), (4, 5), (4, 6), (5, 6) \} $$
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$$ R^{-1} = \{(4, 3), (5, 3), (6, 3), (5, 4), (6, 4), (6, 5) \} $$
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11. Let $A = \{3, 4, 5\}$ and $B = \{4, 5, 6\}$ and let $S$ be the "divides"
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relation. That is, for every ordered pair $(x, y) \in A \times B$,
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$$ x S y \Leftrightarrow x | y $$
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State explicitly which ordered pairs are in $S$ and $S^{-1}$.
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$$ S = \{(3, 6), (4, 4), (5, 5)\} $$
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$$ S^{-1} = \{(6, 3), (4, 4), (5, 5)\} $$
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12.
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a. Suppose a function $F: X \to Y$ is one-to-one but not onto. Is $F^{-1}$ (the
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inverse relation for $F$) a function? Explain your answer.
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No, if $F: X \to Y$ is one-to-one, but not onto, then its inverse relation
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$F^{-1}: Y \to X$ will have some elements in its domain that have not elements
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in the co-domain. More formally:
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$$ \exists y \in Y | (y, x) \notin F^{-1} $$
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which means $F^{-1}$ does not satisfy property 1 for being a function.
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b. Suppose a function $F: X \to Y$ is onto but not one-to-one. Is $F^{-1}$ (the
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inverse relation for $F$) a function? Explain your answer.
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No, if $F: X \to Y$ is onto, but not one-to-one, it follows that its inverse
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relation $F^{-1}: Y \to X$ will have at least one
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$y \in Y | (y, x_1) \in F^{-1} \wedge (y, x_2) \in F^{-1}$.
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This violates property 2 of the definition of a function.
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Draw the directed graphs of the relations defined in 13-18.
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13. Define a relation $R$ on $A = \{0, 1, 2, 3\}$ by
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$R = \{(0, 0), (1, 2), (2, 2)\}$.
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(Done by hand.)
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14. Define a relation $S$ on $B = \{a, b, c, d\}$ by
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$S = \{(a, b), (a, c), (b, c), (d, d)\}$.
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(Done by hand.)
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15. Let $A = \{2, 3, 4, 5, 6, 7, 8\}$ and define a relation $R$ on $A$ as
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follows: For every $x, y \in A$,
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$$ x R y \Leftrightarrow x | y $$
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(Done by hand.)
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16. Let $A = \{5, 6, 7, 8, 9, 10\}$ and define a relation $S$ on $A$ as follows:
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For every $x, y \in A$,
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$$ x S y \Leftrightarrow 2 | (x - y) $$
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(Done by hand.)
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17. Let $A = \{2, 3, 4, 5, 6, 7, 8\}$ and define a relation $T$ on $A$ as
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follows: For every $x, y \in A$,
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$$ x T y \Leftrightarrow 3 | (x - y) $$
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(Done by hand.)
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18. Let $A = \{0, 1, 3, 4, 5, 6\}$ and define a relation $V$ on $A$ as follows:
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For every $x, y \in A$,
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$$ x V y \Leftrightarrow 5 | (x^2 - y^2) $$
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(Done by hand.)
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Exercises 19-20 refer to unions and intersections of relations. Since relations
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are subsets of Cartesian products, their unions and intersections can be
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calculated as for any subsets. Given two relations $R$ and $S$ from $A$ to $B$,
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$$ R \cup S = \{(x, y) \in A \times B | (x, y) \in R \text{ or } (x, y) \in S\} $$
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$$ R \cap S = \{(x, y) \in A \times B | (x, y) \in R \text{ and } (x, y) \in S\} $$
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19. Let $A = \{2, 4\}$ and $B = \{6, 8, 10\}$ and define relations $R$ and $S$
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from $A$ to $B$ as follows: For every $(x, y) \in A \times B$,
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$$ x R y \Leftrightarrow x | y \text{ and } x S y \Leftrightarrow y - 4 = x $$
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State explicitly which ordered pairs are in $A \times B$, $R$, $S$, $R \cup S$,
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and $R \cap S$.
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$$ A \times B = \{(2, 6), (2, 8), (2, 10), (4, 6), (4, 8), (4, 10)\} $$
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$$ R = \{(2, 6), (2, 8), (2, 10), (4, 8)\} $$
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$$ S = \{(2, 6), (4, 8)\} $$
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$$ R \cup S = \{(2, 6), (2, 8), (2, 10), (4, 8)\} = R $$
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$$ R \cap S = \{(2, 6), (4, 8)\} = S $$
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20. Let $A = \{-1, 1, 2, 4\}$ and $B = \{1, 2\}$ and define relations $R$ and
|
|
$S$ from $A$ to $B$ as follows: For every $(x, y) \in A \times B$,
|
|
|
|
$$ x R y \Leftrightarrow |x| = |y| \text{ and } x S y \Leftrightarrow x - y \text{ is even} $$
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|
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State explicitly which ordered pairs are in $A \times B$, $R$, $S$, $R \cup S$,
|
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and $R \cap S$.
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$$ A \times B = \{(-1, 1), (-1, 2), (1, 1), (1, 2), (2, 1), (2, 2), (4, 1), (4, 2)\} $$
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$$ R = \{(-1, 1), (1, 1), (2, 2)\} $$
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$$ S = \{(-1, 1), (1, 1), (2, 2), (4, 2)\} $$
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$$ R \cup S = \{(-1, 1), (1, 1), (2, 2), (4, 2)\} = S $$
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$$ R \cap S = \{(-1, 1), (1, 1), (2, 2)\} = R $$
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21. Define relations $R$ and $S$ on $\mathbb{R}$ as follows:
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$$ R = \{(x, y) \in \mathbb{R} \times \mathbb{R} | x < y\} \text{ and } S = \{(x, y) \in \mathbb{R} \times \mathbb{R} | x = y\}$$
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|
|
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That is, $R$ is the "less than" relation and $S$ is the "equals" relation on
|
|
$\mathbb{R}$. Graph $R$, $S$, $R \cup S$, and $R \cap S$ in the Cartesian plane.
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Think on this and then see appendix b (Page 975).
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22. Define relations $R$ and $S$ on $\mathbb{R}$ as follows:
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|
|
|
$$ R = \{(x, y) \in \mathbb{R} \times \mathbb{R} | x^2 + y^2 = 4\} \text{ and } S = \{(x, y) \in \mathbb{R} \times \mathbb{R} | x = y\} $$
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|
|
|
Graph $R$, $S$, $R \cup S$, and $R \cap S$ in the Cartesian plane.
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|
|
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23. Define relations $R$ and $S$ on $\mathbb{R}$ as follows:
|
|
|
|
$$ R = \{(x, y) \in \mathbb{R} \times \mathbb{R} | y = |x|\} \text{ and } S = \{(x, y) \in \mathbb{R} \times \mathbb{R} | y = 1\} $$
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|
|
|
Graph $R$, $S$, $R \cup S$, and $R \cap S$ in the Cartesian plane.
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|
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|
$R$ is a circle about the origin (with intersections along the axis along
|
|
$(-2, 0), (0, 2), (2, 0), (-2, 0)$). $S$ is a straight diagonal line ascending
|
|
from the left to the right, intersecting the origin $(0, 0)$.
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|
|
|
$R \cup S$ is just the two graphs drawn together.
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|
|
|
$R \cap S$ is only the two points along which the two graphs intersect.
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|
|
|
(Done by hand.)
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|
|
|
24. In Example 8.1.7 consider the query SELECT Patient_ID#, Name FROM S WHERE
|
|
Primary_Diagnosis = X. The response query is the projection onto the first
|
|
two coordinates of the intersection of the database with the set
|
|
$A_1 \times A_2 \times A_3 \times \{X\}$.
|
|
|
|
a. Find the result of the query SELECT Patient_ID#, Name FROM S WHERE
|
|
Primary_Diagnosis = pneumonia.
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|
|
|
(574329, Tak Kurosawa),
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|
|
|
(011985, John Schmidt)
|
|
|
|
b. Find the result of the query SELECT Patient_ID#, Name FROM S WHERE
|
|
Primary_Diagnosis = appendicitis.
|
|
|
|
(466581, Mary Lazars),
|
|
|
|
(778400, Jamal Baskers)
|
|
|
|
---
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Page 526
|
|
|
|
**Exercise Set 8.2**
|
|
|
|
In 1-8, a number of relations are defined on the set $A = \{0, 1, 2, 3\}$. For
|
|
each relation:
|
|
|
|
a. Draw the directed graph.
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|
|
|
b. Determine whether the relation is reflexive.
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|
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|
c. Determine whether the relation is symmetric.
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|
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|
d. Determine whether the relation is transitive.
|
|
|
|
Give a counterexample in each case in which the relation does not satisfy one of
|
|
the properties.
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|
|
|
1. $R_1 = \{(0, 0), (0, 1), (0, 3), (1, 1), (1, 0), (2, 3), (3, 3)\}$
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|
|
|
2. $R_2$ = \{(0, 0), (0, 1), (1, 1), (1, 2), (2, 2), (2, 3)\}
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|
|
|
3. $R_3 = \{(2, 3), (3, 2)\}$
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|
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|
4. $R_4 = \{(1, 2), (2, 1), (1, 3), (3, 1)\}$
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|
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|
5. $R_5 = \{(0, 0), (0, 1), (0, 2), (1, 2)\}$
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|
|
|
6. $R_6 = \{(0, 1), (0, 2)\}$
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|
|
|
7. $R_7 = \{(0, 3), (2, 3)\}$
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|
|
|
8. $R_8 = \{(0, 0), (1, 1)\}$
|
|
|
|
In 9-33, determine whether the given relation is reflexive, symmetric,
|
|
transitive, or none of these. Justify your answers.
|
|
|
|
9. $R$ is the "greater than or equal to" relation on the set of real numbers:
|
|
For every $x, y \in \mathbb{R}$, $x R y \Leftrightarrow x \geq y$.
|
|
|
|
10. $C$ is the circle relation on the set of real numbers: For every
|
|
$x, y \in \mathbb{R}, x C y \Leftrightarrow x^2 + y^2 = 1$.
|
|
|
|
11. $D$ is the relation defined on $\mathbb{R}$ as follows: For every
|
|
$x, y \in \mathbb{R}, x D y \Leftrightarrow xy \geq 0$.
|
|
|
|
12. $E$ is the congruence modulo $4$ relation on $\mathbb{Z}$: For every
|
|
$m, n \in \mathbb{Z}, m E n \Leftrightarrow 4 | (m - n)$.
|
|
|
|
13. $F$ is the congruence modulo $5$ relation on $\mathbb{Z}$: For every
|
|
$m, n \in \mathbb{Z}, m F n \Leftrightarrow 5 | (m - n)$.
|
|
|
|
14. $O$ is the relation defined on $\mathbb{Z}$ as follows: For every
|
|
$m, n \in \mathbb{Z}, m O n \Leftrightarrow m - n \text{ is odd}$.
|
|
|
|
15. $D$ is the "divides" relation on $\mathbb{Z}^+$: For all positive integers
|
|
$m$ and $n$, $m D n \Leftrightarrow m | n$.
|
|
|
|
16. $A$ is the "absolute value" relation on $\mathbb{R}$: For all real numbers
|
|
$x$ and $y$, $x A y \Leftrightarrow |x| = |y|$.
|
|
|
|
17. Recall that a prime number is an integer that is greater than $1$ and has no
|
|
positive integer divisors other than $1$ and itself. (In particular, $1$ is
|
|
not prime.) A relation $P$ is defined on $\mathbb{Z}$ as follows: For every
|
|
$m, n \in \mathbb{Z}, m P n \Leftrightarrow \exists \text{ a prime number } p \text{ such that } p | m \text{ and } p | n$.
|
|
|
|
18. Define a relation $Q$ on $\mathbb{R}$ as follows: For all real numbers $x$
|
|
and $y$, $x Q y \Leftrightarrow x - y$ is rational.
|
|
|
|
19. Define a relation $I$ on $\mathbb{R}$ as follows: For all real numbers $x$
|
|
and $y$, $x I y \Leftrightarrow x - y$ is irrational.
|
|
|
|
20. Let $X = \{a, b, c\}$ and $\mathscr{P}(X)$ be the power set of $X$ (the set
|
|
of all subsets of $X$). A relation $\mathbf{E}$ is defined on
|
|
$\mathscr{P}(X)$ as follows: For every
|
|
$A, B \in \mathscr{P}(X), A \mathbf{E} B \Leftrightarrow \text{ the number of elements in } A \text{ equals the number of elements in } B$.
|
|
|
|
21. Let $X = \{a, b, c\}$ and $\mathscr{P}(X)$ be the power set of $X$. A
|
|
relation $\mathbf{L}$ is defined on $\mathscr{P}(X)$ as follows: For every
|
|
$A, B \in \mathscr{P}(X), A \mathbf{L} B \Leftrightarrow \text{ the number of elements in } A \text{ is less than the number of elements in } B$.
|
|
|
|
22. Let $X = \{a, b, c\}$ and $\mathscr{P}(X)$ be the power set of $X$. A
|
|
relation $\mathbf{N}$ is defined on $\mathscr{P}(X)$ as follows: For every
|
|
$A, B \in \mathscr{P}(X), A \mathbf{N} B \Leftrightarrow \text{ the number of elements in } A \text{ is not equal to the number of elements in } B$.
|
|
|
|
23. Let $X$ be a nonempty set and $\mathscr{P}(X)$ the power set of $X$. Define
|
|
the "subset" relation $\mathbf{S}$ on $\mathscr{P}(X)$ as follows: For every
|
|
$A, B \in \mathscr{P}(X), A \mathbf{S} B \Leftrightarrow A \subseteq B$.
|
|
|
|
24. Let $X$ be a nonempty set and $\mathscr{P}(X)$ the power set of $X$. Define
|
|
the "not equal to" relation $\mathbf{U}$ on $\mathscr{P}(X)$ as follows: For
|
|
every $A, B \in \mathscr{P}(X), A \mathbf{U} B \Leftrightarrow A \neq B$.
|
|
|
|
25. Let $A$ be the set of all strings of _a_'s and _b_'s of length $4$. Define a
|
|
relation $R$ on $A$ as follows: For every
|
|
$s, t \in A, s R t \Leftrightarrow s \text{ has the same first two characters as } t$.
|
|
|
|
26. Let $A$ be the set of all strings of 0's, 1's, and 2's that have length 4
|
|
and for which the sum of the characters in the string is less than or equal
|
|
to 2. Define a relation $R$ on $A$ as follows: For every
|
|
$s, t \in A, s R t \Leftrightarrow \text{ the sum of the characters of } s \text{ equals the sum of the characters of } t$.
|
|
|
|
27. Let $A$ be the set of all English statements. A relation $\mathbf{I}$ is
|
|
defined on $A$ as follows: For every $p, q \in A$,
|
|
|
|
$$ p \mathbf{I} q \Leftrightarrow p \to q \text{ is true} $$
|
|
|
|
28. Let $A = \mathbb{R} \times \mathbb{R}$. A relation $\mathbf{S}$ is defined
|
|
on $A$ as follows: For every $(x_1, y_1)$ and $(x_2, y_2)$ in $A$,
|
|
|
|
$$ (x_1, y_2) \mathbf{S} (x_2, y_2) \Leftrightarrow x_1 = x_2 $$
|
|
|
|
29. Let $A = \mathbb{R} \times \mathbb{R}$. A relation $\mathbf{S}$ is defined
|
|
on $A$ as follows: For every $(x_1, y_1)$ and $(x_2, y_2)$ in $A$,
|
|
|
|
$$ (x_1, y_2) \mathbf{S} (x_2, y_2) \Leftrightarrow y_1 = y_2 $$
|
|
|
|
30. Let $A$ be the "punctured plane"; that is, $A$ is the set of all points in
|
|
the Cartesian plane except the origin $(0, 0)$. A relation $R$ is defined on
|
|
$A$ as follows: For every $p_1$ and $p_2$ in $A$,
|
|
$p_1 R p_2 \Leftrightarrow p_1 \text{ and } p_2 \text{ lie on the same half line emanating from the origin}$.
|
|
|
|
31. Let $A$ be the set of people living in the world today. A relation $R$ is
|
|
defined on $A$ as follows: For all people $p$ and $q$ in $A$,
|
|
|
|
$$ p R q \Leftrightarrow p \text{ lives within 100 miles of } q $$
|
|
|
|
32. Let $A$ be the set of all lines in the plane. A relation $R$ is defined on
|
|
$A$ as follows: For every $l_1$ and $l_2$ in $A$,
|
|
$l_1 R l_2 \Leftrightarrow l_1 \text{ is parallel to } l_2$. (Assume that a
|
|
line is parallel to itself.)
|
|
|
|
33. Let $A$ be the set of all lines in the plane. A relation $R$ is defined on
|
|
$A$ as follows: For every $l_1$ and $l_2$ in $A$,
|
|
|
|
$$ l_1 R l_2 \Leftrightarrow l_1 \text{ is perpendicular to } l_2 $$
|
|
|
|
In 34-36, assume that $R$ is a relation on a set $A$. Prove or disprove each
|
|
statement.
|
|
|
|
34. If $R$ is reflexive, then $R^{-1}$ is reflexive.
|
|
|
|
35. If $R$ is symmetric, then $R^{-1}$ is symmetric.
|
|
|
|
36. If $R$ is transitive, then $R^{-1}$ is transitive.
|
|
|
|
In 37-42, assume that $R$ and $S$ are relations on a set $A$. Prove or disprove
|
|
each statement.
|
|
|
|
37. If $R$ and $S$ are reflexive, is $R \cap S$ reflexive? Why?
|
|
|
|
38. If $R$ and $S$ are symmetric, is $R \cap S$ symmetric? Why?
|
|
|
|
39. If $R$ and $S$ are transitive, is $R \cap S$ transitive? Why?
|
|
|
|
40. If $R$ and $S$ are reflexive, is $R \cup S$ reflexive? Why?
|
|
|
|
41. If $R$ and $S$ are symmetric, is $R \cup S$ symmetric? Why?
|
|
|
|
42. If $R$ and $S$ are transitive, is $R \cup S$ transitive? Why?
|
|
|
|
In 43-50, the following definitions are used: A relation on a set $A$ is defined
|
|
to be
|
|
|
|
irreflexive if, and only if, for every $x \in A, x \cancel{R} x$;
|
|
|
|
asymmetric if, and only if, for every $x, y \in A$ if $x R y$ then
|
|
$y \cancel{R} x$;
|
|
|
|
intransitive if, and only if, for every $x, y, z \in A$, if $x R y$ and $y R z$
|
|
then $x \cancel{R} z$.
|
|
|
|
For each of the relations in the referenced exercise, determine whether the
|
|
relation is irreflexive, asymmetric, intransitive, or none of these.
|
|
|
|
43. Exercise 1
|
|
|
|
44. Exercise 2
|
|
|
|
45. Exercise 3
|
|
|
|
46. Exercise 4
|
|
|
|
47. Exercise 5
|
|
|
|
48. Exercise 6
|
|
|
|
49. Exercise 7
|
|
|
|
50. Exercise 8
|
|
|
|
In 51-53, $R$, $S$, and $T$ are relations defined on $A = \{0, 1, 2, 3\}$.
|
|
|
|
51. Let $R = \{(0, 1), (0, 2), (1, 1), (1, 3), (2, 2), (3, 0)\}$.
|
|
|
|
Find $R^t$, the transitive closure of $R$.
|
|
|
|
52. Let $S = \{(0, 0), (0, 3), (1, 0), (1, 2), (2, 0), (3, 2)\}$.
|
|
|
|
Find $S^t$, the transitive closure of $S$.
|
|
|
|
53. Let $T = \{(0, 2), (1, 0), (2, 3), (3, 1)\}$.
|
|
|
|
Find $T^t$, the transitive closure of $T$.
|
|
|
|
54. Write a computer algorithm to test whether a relation $R$ defined on a
|
|
finite set $A$ is reflexive, where
|
|
|
|
$$ A = \{a[1], a[2], \dots, a[n]\} $$
|
|
|
|
55. Write a computer algorithm to test whether a relation $R$ defined on a
|
|
finite set $A$ is symmetric, where
|
|
|
|
$$ A = \{a[1], a[2], \dots, a[n]\} $$
|
|
|
|
56. Write a computer algorithm to test whether a relation $R$ defined on a
|
|
finite set $A$ is transitive, where
|
|
|
|
$$ A = \{a[1], a[2], \dots, a[n]\} $$
|