1.2.E: Problems on Relations and Mappings (Exercises)
- Page ID
- 22252
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)
( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\id}{\mathrm{id}}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\kernel}{\mathrm{null}\,}\)
\( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\)
\( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\)
\( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)
\( \newcommand{\vectorA}[1]{\vec{#1}} % arrow\)
\( \newcommand{\vectorAt}[1]{\vec{\text{#1}}} % arrow\)
\( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vectorC}[1]{\textbf{#1}} \)
\( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)
\( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)
\( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)For the relations specified in Problem 7 of §§1-3, find \(D_{R}, D_{R}^{\prime},\) and \(R^{-1}\). Also, find \(R[A]\) and \(R^{-1}[A]\) if
\[
\begin{array}{ll}{\text { (a) } A=\left\{\frac{1}{2}\right\} ;} & {\text { (b) } A=\{1\}} \\ {\text { (c) } A=\{0\} ;} & {\text { (d) } A=\emptyset \text { ; }} \\ {\text { (e) } A=\{0,3,-15\} ;} & {\text { (f) } A=\{3,4,7,0,-1,6\}} \\ {\text { (g) } A=\{x |-20<x<5\}}\end{array}
\]
Prove that if \(A \subseteq B\), then \(R[A] \subseteq R[B] .\) Disprove the converse by a counterexample.
Prove that
(i) \(R[A \cup B]=R[A] \cup R[B]\);
(ii) \(R[A \cap B] \subseteq R[A] \cap R[B]\);
(iii) \(R[A-B] \supseteq R[A]-R[B]\).
Disprove reverse inclusions in (ii) and (iii) by examples. Do (i) and (ii) with \(A, B\) replaced by an arbitrary set family \(\left\{A_{i} | i \in I\right\}\).
Under which conditions are the following statements true?
\[
\begin{array}{cc} \text{(i) } R[x] = \emptyset ; & \text{(ii) } R^{-1}[x] = \emptyset ; \\ \text{(iii) } R[A] = \emptyset ; & \text{(iv) } R^{-1}[A] = \emptyset ; \end{array}
\]
Let \(f : N \rightarrow N(N=\{\text { naturals }\}) .\) For each of the following functions, specify \(f[N],\) i.e., \(D_{f}^{\prime},\) and determine whether \(f\) is one to one and onto \(N,\) given that for all \(x \in N\),
\[
\begin{array}{ll}{\text { (i) } f(x)=x^{3} ;} & {\text { (ii) } f(x)=1 ; \quad \text { (iii) } f(x)=|x|+3} \\ {\text { (iv) } f(x)=x^{2} ;} & {(\mathrm{v}) f(x)=4 x+5}\end{array}
\]
Do all this also if \(N\) denotes
(a) the set of all integers;
(b) the set of all reals.
Prove that for any mapping \(f\) and any sets \(A, B, A_{i}(i \in I)\),
(a) \(f^{-1}[A \cup B]=f^{-1}[A] \cup f^{-1}[B]\);
(b) \(f^{-1}[A \cap B]=f^{-1}[A] \cap f^{-1}[B]\);
(c) \(f^{-1}[A-B]=f^{-1}[A]-f^{-1}[B]\);
(d) \(f^{-1}\left[\bigcup_{i} A_{i}\right]=\bigcup_{i} f^{-1}\left[A_{i}\right]\);
(e) \(f^{-1}\left[\bigcap_{i} A_{i}\right]=\bigcap_{i} f^{-1}\left[A_{i}\right]\).
Compare with Problem 3.
[Hint: First verify that \(x \in f^{-1}[A]\) iff \(x \in D_{f}\) and \(f(x) \in A . ]\)
Let \(f\) be a map. Prove that
(a) \(f\left[f^{-1}[A]\right] \subseteq A\);
(b) \(f\left[f^{-1}[A]\right]=A\) if \(A \subseteq D_{f}^{\prime}\);
(c) if \(A \subseteq D_{f}\) and \(f\) is one to one, \(A=f^{-1}[f[A]]\)/
Is \(f[A] \cap B \subseteq f\left[A \cap f^{-1}[B]\right] ?\)
Is \(R\) an equivalence relation on the set \(J\) of all integers, and, if so, what are the \(R\) -classes, if
(a) \(R=\{(x, y) | x-y \text { is divisible by a fixed } n\}\);
(b) \(R=\{(x, y) | x-y \text { is odd }\}\);
(c) \(R=\{(x, y) | x-y \text { is a prime }\}\).
\((x, y, n \text { denote integers.) }\)
Is any relation in Problem 7 of §§1-3 reflexive? Symmetric? Transitive?
10. Show by examples that \(R\) may be
(a) reflexive and symmetric, without being transitive;
(b) reflexive and transitive without being symmetric.
Does symmetry plus transitivity imply reflexivity? Give a proof or counterexample.