<exercises xml:id="exercises-integers">
<title>Exercises</title>
<exercise number="1">
<statement>
<p>Prove that
<md>1^2 + 2^2 + \cdots + n^2 = \frac{n(n + 1)(2n + 1)}{6}</md>
for <m>n \in {\mathbb N}</m>.</p>
</statement>
<answer>
<p>The base case, <m>S(1): [1(1 + 1)(2(1) + 1)]/6 = 1 = 1^2</m> is true.</p>
<p>Assume that <m>S(k): 1^2 + 2^2 + \cdots + k^2 = [k(k + 1)(2k + 1)]/6</m> is true. Then<md>
<mrow>1^2 + 2^2 + \cdots + k^2 + (k + 1)^2 & = [k(k + 1)(2k + 1)]/6 + (k + 1)^2</mrow>
<mrow>& = [(k + 1)((k + 1) + 1)(2(k + 1) + 1)]/6,</mrow>
</md>and so <m>S(k + 1)</m> is true. Thus, <m>S(n)</m> is true for all positive integers <m>n</m>.</p>
</answer>
</exercise>
<exercise number="2">
<statement>
<p>Prove that
<md>1^3 + 2^3 + \cdots + n^3 = \frac{n^2(n + 1)^2}{4}</md>
for <m>n \in {\mathbb N}</m>.</p>
</statement>
</exercise>
<exercise number="3">
<statement>
<p>Prove that <m>n! \gt 2^n</m> for <m>n \geq 4</m>.</p>
</statement>
<answer>
<p>The base case, <m>S(4): 4! = 24 \gt 16 =2^4</m> is true. Assume <m>S(k): k! \gt 2^k</m> is true. Then <m>(k + 1)! = k! (k + 1) \gt 2^k \cdot 2 = 2^{k + 1}</m>, so <m>S(k + 1)</m> is true. Thus, <m>S(n)</m> is true for all positive integers <m>n</m>.</p>
</answer>
</exercise>
<exercise number="4">
<statement>
<p>Prove that
<md>x + 4x + 7x + \cdots + (3n - 2)x = \frac{n(3n - 1)x}{2}</md>
for <m>n \in {\mathbb N}</m>.</p>
</statement>
</exercise>
<exercise number="5">
<statement>
<p>Prove that <m>10^{n + 1} + 10^n + 1</m> is divisible by 3 for <m>n \in {\mathbb N}</m>.</p>
</statement>
</exercise>
<exercise number="6">
<statement>
<p>Prove that <m>4 \cdot 10^{2n} + 9 \cdot 10^{2n - 1} + 5</m> is divisible by 99 for <m>n \in {\mathbb N}</m>.</p>
</statement>
</exercise>
<exercise number="7">
<statement>
<p>Show that
<md>\sqrt[n]{a_1 a_2 \cdots a_n} \leq \frac{1}{n} \sum_{k = 1}^{n} a_k.</md></p>
</statement>
</exercise>
<exercise number="8">
<statement>
<p>Use induction to prove that <m>1 + 2 + 2^2 + \cdots + 2^n = 2^{n + 1} - 1</m> for <m>n \in {\mathbb N}</m>.</p>
</statement>
</exercise>
<exercise number="9">
<statement>
<p>Prove the Leibniz rule for <m>f^{(n)} (x)</m>, where <m>f^{(n)}</m> is the <m>n</m>th derivative of <m>f</m>; that is, show that
<md>(fg)^{(n)}(x) = \sum_{k = 0}^{n} \binom{n}{k} f^{(k)}(x) g^{(n - k)}(x).</md></p>
</statement>
<hint>
<p>Follow the proof in <xref ref="example-integers-binomial-theorem"/>.</p>
</hint>
</exercise>
<exercise number="10">
<statement>
<p>Prove that
<md>\frac{1}{2}+ \frac{1}{6} + \cdots + \frac{1}{n(n + 1)} = \frac{n}{n + 1} </md>
for <m>n \in {\mathbb N}</m>.</p>
</statement>
</exercise>
<exercise number="11">
<statement>
<p>If <m>x</m> is a nonnegative real number, then show that <m>(1 + x)^n - 1 \geq nx</m> for <m>n = 0, 1, 2, \ldots</m>.</p>
</statement>
<hint>
<p>The base case, <m>S(0): (1 + x)^0 - 1 = 0 \geq 0 = 0 \cdot x</m> is true. Assume <m>S(k): (1 + x)^k -1 \geq kx</m> is true. Then<md>
<mrow>(1 + x)^{k + 1} - 1 & = (1 + x)(1 + x)^k -1</mrow>
<mrow>& = (1 + x)^k + x(1 + x)^k - 1</mrow>
<mrow>& \geq kx + x(1 + x)^k</mrow>
<mrow>& \geq kx + x</mrow>
<mrow>& = (k + 1)x,</mrow>
</md>so <m>S(k + 1)</m> is true. Therefore, <m>S(n)</m> is true for all positive integers <m>n</m>.</p>
</hint>
</exercise>
<exercise number="12">
<title>Power Sets</title>
<statement>
<p>Let <m>X</m> be a set. Define the <term>power set</term> of <m>X</m>, denoted <m>{\mathcal P}(X)</m>, to be the set of all subsets of <m>X</m>. <notation><usage><m>\mathcal P(X)</m></usage><description>power set of <m>X</m></description></notation> For example,
<md>{\mathcal P}( \{a, b\} ) = \{ \emptyset, \{a\}, \{b\}, \{a, b\} \}.</md>
For every positive integer <m>n</m>, show that a set with exactly <m>n</m> elements has a power set with exactly <m>2^n</m> elements.</p>
</statement>
</exercise>
<exercise number="13">
<statement>
<p>Prove that the two principles of mathematical induction stated in <xref ref="section-math-induction"/> are equivalent.</p>
</statement>
</exercise>
<exercise number="14">
<statement>
<p>Show that the Principle of Well-Ordering for the natural numbers implies that 1 is the smallest natural number. Use this result to show that the Principle of Well-Ordering implies the Principle of Mathematical Induction; that is, show that if <m>S \subset {\mathbb N}</m> such that <m>1 \in S</m> and <m>n + 1 \in S</m> whenever <m>n \in S</m>, then <m>S = {\mathbb N}</m>.</p>
</statement>
</exercise>
<exercise number="15">
<statement>
<p>For each of the following pairs of numbers <m>a</m> and <m>b</m>, calculate <m>\gcd(a,b)</m> and find integers <m>r</m> and <m>s</m> such that <m>\gcd(a,b) = ra + sb</m>.</p>
<ol cols="2">
<li><p>14 and 39</p></li>
<li><p>234 and 165</p></li>
<li><p>1739 and 9923</p></li>
<li><p>471 and 562</p></li>
<li><p>23,771 and 19,945</p></li>
<li><p><m>-4357</m> and 3754</p></li>
</ol>
</statement>
</exercise>
<exercise number="16" xml:id="exercise-integers-gcd-1">
<statement>
<p>Let <m>a</m> and <m>b</m> be nonzero integers. If there exist integers <m>r</m> and <m>s</m> such that <m>ar + bs =1</m>, show that <m>a</m> and <m>b</m> are relatively prime.</p>
</statement>
</exercise>
<!-- Todo: Check references to the Fibonacci numbers -->
<exercise number="17">
<title>Fibonacci Numbers</title>
<statement>
<p>The Fibonacci numbers are
<md>1, 1, 2, 3, 5, 8, 13, 21, \ldots.</md>
We can define them inductively by <m>f_1 = 1</m>, <m>f_2 = 1</m>, and <m>f_{n + 2} = f_{n + 1} + f_n</m> for <m>n \in {\mathbb N}</m>.</p>
<ol>
<li xml:id="fn_ineq"><p>Prove that <m>f_n \lt 2^n</m>.</p></li>
<li xml:id="fn_prod"><p>Prove that <m>f_{n + 1} f_{n - 1} = f^2_n + (-1)^n</m>, <m>n \geq 2</m>.</p></li>
<li xml:id="fn_formula"><p>Prove that <m>f_n = [(1 + \sqrt{5}\, )^n - (1 - \sqrt{5}\, )^n]/ 2^n \sqrt{5}</m>.</p></li>
<li xml:id="fn_ratio"><p>Show that <m>\lim_{n \rightarrow \infty} f_n / f_{n + 1} = (\sqrt{5} - 1)/2</m>.</p></li>
<li xml:id="fn_coprime"><p>Prove that <m>f_n</m> and <m>f_{n + 1}</m> are relatively prime.</p></li>
</ol>
</statement>
<hint>
<p>For <xref ref="fn_ineq"/> and <xref ref="fn_prod"/> use mathematical induction.
<xref ref="fn_formula"/> Show that <m>f_1 = 1</m>, <m>f_2 = 1</m>, and <m>f_{n + 2} = f_{n + 1} + f_n</m>.
<xref ref="fn_ratio"/> Use part <xref ref="fn_formula"/>.
<xref ref="fn_coprime"/> Use part <xref ref="fn_prod"/> and <xref ref="exercise-integers-gcd-1"/>.</p>
</hint>
</exercise>
<exercise number="18">
<statement>
<p>Let <m>a</m> and <m>b</m> be integers such that <m>\gcd(a,b) = 1</m>. Let <m>r</m> and <m>s</m> be integers such that <m>ar + bs =1</m>. Prove that
<md>\gcd(a,s) = \gcd(r,b) = \gcd(r,s) = 1.</md></p>
</statement>
</exercise>
<exercise number="19">
<statement>
<p>Let <m>x, y \in {\mathbb N}</m> be relatively prime. If <m>xy</m> is a perfect square, prove that <m>x</m> and <m>y</m> must both be perfect squares.</p>
</statement>
<hint>
<p>Use the Fundamental Theorem of Arithmetic.</p>
</hint>
</exercise>
<exercise number="20">
<statement>
<p>Using the division algorithm, show that every perfect square is of the form <m>4k</m> or <m>4k + 1</m> for some nonnegative integer <m>k</m>.</p>
</statement>
</exercise>
<exercise number="21">
<statement>
<p>Suppose that <m>a, b, r, s</m> are pairwise relatively prime and that<md>
<mrow>a^2 + b^2 & = r^2</mrow>
<mrow>a^2 - b^2 & = s^2.</mrow>
</md></p>
<p>Prove that <m>a</m>, <m>r</m>, and <m>s</m> are odd and <m>b</m> is even.</p>
</statement>
</exercise>
<exercise number="22">
<statement>
<p>Let <m>n \in {\mathbb N}</m>. Use the division algorithm to prove that every integer is congruent mod <m>n</m> to precisely one of the integers <m>0, 1, \ldots, n-1</m>. Conclude that if <m>r</m> is an integer, then there is exactly one <m>s</m> in <m>{\mathbb Z}</m> such that <m>0 \leq s \lt n</m> and <m>[r] = [s]</m>. Hence, the integers are indeed partitioned by congruence mod <m>n</m>.</p>
</statement>
</exercise>
<exercise number="23">
<statement>
<p>Define the <term>least common multiple</term> of two nonzero integers <m>a</m> and <m>b</m>, denoted by <m>\lcm(a,b)</m>, to be the nonnegative integer <m>m</m> such that both <m>a</m> and <m>b</m> divide <m>m</m>, and if <m>a</m> and <m>b</m> divide any other integer <m>n</m>, then <m>m</m> also divides <m>n</m>. <notation><usage><m>\lcm(m,n)</m></usage><description>the least common multiple of <m>m</m> and <m>n</m></description></notation> Prove that any two integers <m>a</m> and <m>b</m> have a unique least common multiple.</p>
</statement>
<hint>
<p>Let <m>S = \{s \in {\mathbb N} : a \mid s</m>, <m>b \mid s \}</m>. Then <m>S \neq \emptyset</m>, since <m>|ab| \in S</m>. By the Principle of Well-Ordering, <m>S</m> contains a least element <m>m</m>. To show uniqueness, suppose that <m>a \mid n</m> and <m>b \mid n</m> for some <m>n \in {\mathbb N}</m>. By the division algorithm, there exist unique integers <m>q</m> and <m>r</m> such that <m>n = mq + r</m>, where <m>0 \leq r \lt m</m>. Since <m>a</m> and <m>b</m> divide both <m>m</m>, and <m>n</m>, it must be the case that <m>a</m> and <m>b</m> both divide <m>r</m>. Thus, <m>r = 0</m> by the minimality of <m>m</m>. Therefore, <m>m \mid n</m>.</p>
</hint>
</exercise>
<exercise number="24" xml:id="exercise-integers-lcm-gcd">
<statement>
<p>If <m>d= \gcd(a, b)</m> and <m>m = \lcm(a, b)</m>, prove that <m>dm = |ab|</m>.</p>
</statement>
</exercise>
<exercise number="25">
<statement>
<p>Show that <m>\lcm(a,b) = ab</m> if and only if <m>\gcd(a,b) = 1</m>.</p>
</statement>
</exercise>
<exercise number="26">
<statement>
<p>Prove that <m>\gcd(a,c) = \gcd(b,c) =1</m> if and only if <m>\gcd(ab,c) = 1</m> for integers <m>a</m>, <m>b</m>, and <m>c</m>.</p>
</statement>
</exercise>
<exercise number="27">
<statement>
<p>Let <m>a, b, c \in {\mathbb Z}</m>. Prove that if <m>\gcd(a,b) = 1</m> and <m>a \mid bc</m>, then <m>a \mid c</m>. </p>
</statement>
<hint>
<p>Since <m>\gcd(a,b) = 1</m>, there exist integers <m>r</m> and <m>s</m> such that <m>ar + bs = 1</m>. Thus, <m>acr + bcs = c</m>. Since <m>a</m> divides both <m>bc</m> and itself, <m>a</m> must divide <m>c</m>.</p>
</hint>
</exercise>
<exercise number="28">
<statement>
<p>Let <m>p \geq 2</m>. Prove that if <m>2^p - 1</m> is prime, then <m>p</m> must also be prime.</p>
</statement>
</exercise>
<exercise number="29">
<statement>
<p>Prove that there are an infinite number of primes of the form <m>6n + 5</m>. </p>
</statement>
<hint>
<p>Every prime must be of the form 2, 3, <m>6n + 1</m>, or <m>6n + 5</m>. Suppose there are only finitely many primes of the form <m>6k + 5</m>.</p>
</hint>
</exercise>
<exercise number="30">
<statement>
<p>Prove that there are an infinite number of primes of the form <m>4n - 1</m>.</p>
</statement>
</exercise>
<exercise number="31">
<statement>
<p>Using the fact that 2 is prime, show that there do not exist integers <m>p</m> and <m>q</m> such that <m>p^2 = 2 q^2</m>. Demonstrate that therefore <m>\sqrt{2}</m> cannot be a rational number.</p>
</statement>
</exercise>
</exercises>
Exercises 2.4 Exercises
View Source for exercises
1.
View Source for exercise
<exercise number="1">
<statement>
<p>Prove that
<md>1^2 + 2^2 + \cdots + n^2 = \frac{n(n + 1)(2n + 1)}{6}</md>
for <m>n \in {\mathbb N}</m>.</p>
</statement>
<answer>
<p>The base case, <m>S(1): [1(1 + 1)(2(1) + 1)]/6 = 1 = 1^2</m> is true.</p>
<p>Assume that <m>S(k): 1^2 + 2^2 + \cdots + k^2 = [k(k + 1)(2k + 1)]/6</m> is true. Then<md>
<mrow>1^2 + 2^2 + \cdots + k^2 + (k + 1)^2 & = [k(k + 1)(2k + 1)]/6 + (k + 1)^2</mrow>
<mrow>& = [(k + 1)((k + 1) + 1)(2(k + 1) + 1)]/6,</mrow>
</md>and so <m>S(k + 1)</m> is true. Thus, <m>S(n)</m> is true for all positive integers <m>n</m>.</p>
</answer>
</exercise>
Prove that
\begin{equation*}
1^2 + 2^2 + \cdots + n^2 = \frac{n(n + 1)(2n + 1)}{6}
\end{equation*}
for \(n \in {\mathbb N}\text{.}\)
Answer.
View Source for answer
<answer>
<p>The base case, <m>S(1): [1(1 + 1)(2(1) + 1)]/6 = 1 = 1^2</m> is true.</p>
<p>Assume that <m>S(k): 1^2 + 2^2 + \cdots + k^2 = [k(k + 1)(2k + 1)]/6</m> is true. Then<md>
<mrow>1^2 + 2^2 + \cdots + k^2 + (k + 1)^2 & = [k(k + 1)(2k + 1)]/6 + (k + 1)^2</mrow>
<mrow>& = [(k + 1)((k + 1) + 1)(2(k + 1) + 1)]/6,</mrow>
</md>and so <m>S(k + 1)</m> is true. Thus, <m>S(n)</m> is true for all positive integers <m>n</m>.</p>
</answer>
The base case, \(S(1): [1(1 + 1)(2(1) + 1)]/6 = 1 = 1^2\) is true.
Assume that \(S(k): 1^2 + 2^2 + \cdots + k^2 = [k(k + 1)(2k + 1)]/6\) is true. Then
\begin{align*}
1^2 + 2^2 + \cdots + k^2 + (k + 1)^2 & = [k(k + 1)(2k + 1)]/6 + (k + 1)^2\\
& = [(k + 1)((k + 1) + 1)(2(k + 1) + 1)]/6,
\end{align*}
and so \(S(k + 1)\) is true. Thus, \(S(n)\) is true for all positive integers \(n\text{.}\)
2.
View Source for exercise
<exercise number="2">
<statement>
<p>Prove that
<md>1^3 + 2^3 + \cdots + n^3 = \frac{n^2(n + 1)^2}{4}</md>
for <m>n \in {\mathbb N}</m>.</p>
</statement>
</exercise>
Prove that
\begin{equation*}
1^3 + 2^3 + \cdots + n^3 = \frac{n^2(n + 1)^2}{4}
\end{equation*}
for \(n \in {\mathbb N}\text{.}\)
3.
View Source for exercise
<exercise number="3">
<statement>
<p>Prove that <m>n! \gt 2^n</m> for <m>n \geq 4</m>.</p>
</statement>
<answer>
<p>The base case, <m>S(4): 4! = 24 \gt 16 =2^4</m> is true. Assume <m>S(k): k! \gt 2^k</m> is true. Then <m>(k + 1)! = k! (k + 1) \gt 2^k \cdot 2 = 2^{k + 1}</m>, so <m>S(k + 1)</m> is true. Thus, <m>S(n)</m> is true for all positive integers <m>n</m>.</p>
</answer>
</exercise>
Prove that \(n! \gt 2^n\) for \(n \geq 4\text{.}\)
Answer.
View Source for answer
<answer>
<p>The base case, <m>S(4): 4! = 24 \gt 16 =2^4</m> is true. Assume <m>S(k): k! \gt 2^k</m> is true. Then <m>(k + 1)! = k! (k + 1) \gt 2^k \cdot 2 = 2^{k + 1}</m>, so <m>S(k + 1)</m> is true. Thus, <m>S(n)</m> is true for all positive integers <m>n</m>.</p>
</answer>
The base case, \(S(4): 4! = 24 \gt 16 =2^4\) is true. Assume \(S(k): k! \gt 2^k\) is true. Then \((k + 1)! = k! (k + 1) \gt 2^k \cdot 2 = 2^{k + 1}\text{,}\) so \(S(k + 1)\) is true. Thus, \(S(n)\) is true for all positive integers \(n\text{.}\)
4.
View Source for exercise
<exercise number="4">
<statement>
<p>Prove that
<md>x + 4x + 7x + \cdots + (3n - 2)x = \frac{n(3n - 1)x}{2}</md>
for <m>n \in {\mathbb N}</m>.</p>
</statement>
</exercise>
Prove that
\begin{equation*}
x + 4x + 7x + \cdots + (3n - 2)x = \frac{n(3n - 1)x}{2}
\end{equation*}
for \(n \in {\mathbb N}\text{.}\)
5.
View Source for exercise
<exercise number="5">
<statement>
<p>Prove that <m>10^{n + 1} + 10^n + 1</m> is divisible by 3 for <m>n \in {\mathbb N}</m>.</p>
</statement>
</exercise>
Prove that \(10^{n + 1} + 10^n + 1\) is divisible by 3 for \(n \in {\mathbb N}\text{.}\)
6.
View Source for exercise
<exercise number="6">
<statement>
<p>Prove that <m>4 \cdot 10^{2n} + 9 \cdot 10^{2n - 1} + 5</m> is divisible by 99 for <m>n \in {\mathbb N}</m>.</p>
</statement>
</exercise>
Prove that \(4 \cdot 10^{2n} + 9 \cdot 10^{2n - 1} + 5\) is divisible by 99 for \(n \in {\mathbb N}\text{.}\)
7.
View Source for exercise
<exercise number="7">
<statement>
<p>Show that
<md>\sqrt[n]{a_1 a_2 \cdots a_n} \leq \frac{1}{n} \sum_{k = 1}^{n} a_k.</md></p>
</statement>
</exercise>
Show that
\begin{equation*}
\sqrt[n]{a_1 a_2 \cdots a_n} \leq \frac{1}{n} \sum_{k = 1}^{n} a_k.
\end{equation*}
8.
View Source for exercise
<exercise number="8">
<statement>
<p>Use induction to prove that <m>1 + 2 + 2^2 + \cdots + 2^n = 2^{n + 1} - 1</m> for <m>n \in {\mathbb N}</m>.</p>
</statement>
</exercise>
Use induction to prove that \(1 + 2 + 2^2 + \cdots + 2^n = 2^{n + 1} - 1\) for \(n \in {\mathbb N}\text{.}\)
9.
View Source for exercise
<exercise number="9">
<statement>
<p>Prove the Leibniz rule for <m>f^{(n)} (x)</m>, where <m>f^{(n)}</m> is the <m>n</m>th derivative of <m>f</m>; that is, show that
<md>(fg)^{(n)}(x) = \sum_{k = 0}^{n} \binom{n}{k} f^{(k)}(x) g^{(n - k)}(x).</md></p>
</statement>
<hint>
<p>Follow the proof in <xref ref="example-integers-binomial-theorem"/>.</p>
</hint>
</exercise>
Prove the Leibniz rule for \(f^{(n)} (x)\text{,}\) where \(f^{(n)}\) is the \(n\)th derivative of \(f\text{;}\) that is, show that
\begin{equation*}
(fg)^{(n)}(x) = \sum_{k = 0}^{n} \binom{n}{k} f^{(k)}(x) g^{(n - k)}(x).
\end{equation*}
Hint.
View Source for hint
<hint>
<p>Follow the proof in <xref ref="example-integers-binomial-theorem"/>.</p>
</hint>
Follow the proof in Example 2.1.4.
10.
View Source for exercise
<exercise number="10">
<statement>
<p>Prove that
<md>\frac{1}{2}+ \frac{1}{6} + \cdots + \frac{1}{n(n + 1)} = \frac{n}{n + 1} </md>
for <m>n \in {\mathbb N}</m>.</p>
</statement>
</exercise>
Prove that
\begin{equation*}
\frac{1}{2}+ \frac{1}{6} + \cdots + \frac{1}{n(n + 1)} = \frac{n}{n + 1}
\end{equation*}
for \(n \in {\mathbb N}\text{.}\)
11.
View Source for exercise
<exercise number="11">
<statement>
<p>If <m>x</m> is a nonnegative real number, then show that <m>(1 + x)^n - 1 \geq nx</m> for <m>n = 0, 1, 2, \ldots</m>.</p>
</statement>
<hint>
<p>The base case, <m>S(0): (1 + x)^0 - 1 = 0 \geq 0 = 0 \cdot x</m> is true. Assume <m>S(k): (1 + x)^k -1 \geq kx</m> is true. Then<md>
<mrow>(1 + x)^{k + 1} - 1 & = (1 + x)(1 + x)^k -1</mrow>
<mrow>& = (1 + x)^k + x(1 + x)^k - 1</mrow>
<mrow>& \geq kx + x(1 + x)^k</mrow>
<mrow>& \geq kx + x</mrow>
<mrow>& = (k + 1)x,</mrow>
</md>so <m>S(k + 1)</m> is true. Therefore, <m>S(n)</m> is true for all positive integers <m>n</m>.</p>
</hint>
</exercise>
If \(x\) is a nonnegative real number, then show that \((1 + x)^n - 1 \geq nx\) for \(n = 0, 1, 2, \ldots\text{.}\)
Hint.
View Source for hint
<hint>
<p>The base case, <m>S(0): (1 + x)^0 - 1 = 0 \geq 0 = 0 \cdot x</m> is true. Assume <m>S(k): (1 + x)^k -1 \geq kx</m> is true. Then<md>
<mrow>(1 + x)^{k + 1} - 1 & = (1 + x)(1 + x)^k -1</mrow>
<mrow>& = (1 + x)^k + x(1 + x)^k - 1</mrow>
<mrow>& \geq kx + x(1 + x)^k</mrow>
<mrow>& \geq kx + x</mrow>
<mrow>& = (k + 1)x,</mrow>
</md>so <m>S(k + 1)</m> is true. Therefore, <m>S(n)</m> is true for all positive integers <m>n</m>.</p>
</hint>
The base case, \(S(0): (1 + x)^0 - 1 = 0 \geq 0 = 0 \cdot x\) is true. Assume \(S(k): (1 + x)^k -1 \geq kx\) is true. Then
\begin{align*}
(1 + x)^{k + 1} - 1 & = (1 + x)(1 + x)^k -1\\
& = (1 + x)^k + x(1 + x)^k - 1\\
& \geq kx + x(1 + x)^k\\
& \geq kx + x\\
& = (k + 1)x,
\end{align*}
so \(S(k + 1)\) is true. Therefore, \(S(n)\) is true for all positive integers \(n\text{.}\)
12. Power Sets.
View Source for exercise
<exercise number="12">
<title>Power Sets</title>
<statement>
<p>Let <m>X</m> be a set. Define the <term>power set</term> of <m>X</m>, denoted <m>{\mathcal P}(X)</m>, to be the set of all subsets of <m>X</m>. <notation><usage><m>\mathcal P(X)</m></usage><description>power set of <m>X</m></description></notation> For example,
<md>{\mathcal P}( \{a, b\} ) = \{ \emptyset, \{a\}, \{b\}, \{a, b\} \}.</md>
For every positive integer <m>n</m>, show that a set with exactly <m>n</m> elements has a power set with exactly <m>2^n</m> elements.</p>
</statement>
</exercise>
Let \(X\) be a set. Define the power set of \(X\text{,}\) denoted \({\mathcal P}(X)\text{,}\) to be the set of all subsets of \(X\text{.}\) For example,
\begin{equation*}
{\mathcal P}( \{a, b\} ) = \{ \emptyset, \{a\}, \{b\}, \{a, b\} \}.
\end{equation*}
For every positive integer \(n\text{,}\) show that a set with exactly \(n\) elements has a power set with exactly \(2^n\) elements.
13.
View Source for exercise
<exercise number="13">
<statement>
<p>Prove that the two principles of mathematical induction stated in <xref ref="section-math-induction"/> are equivalent.</p>
</statement>
</exercise>
Prove that the two principles of mathematical induction stated in Section 2.1 are equivalent.
14.
View Source for exercise
<exercise number="14">
<statement>
<p>Show that the Principle of Well-Ordering for the natural numbers implies that 1 is the smallest natural number. Use this result to show that the Principle of Well-Ordering implies the Principle of Mathematical Induction; that is, show that if <m>S \subset {\mathbb N}</m> such that <m>1 \in S</m> and <m>n + 1 \in S</m> whenever <m>n \in S</m>, then <m>S = {\mathbb N}</m>.</p>
</statement>
</exercise>
Show that the Principle of Well-Ordering for the natural numbers implies that 1 is the smallest natural number. Use this result to show that the Principle of Well-Ordering implies the Principle of Mathematical Induction; that is, show that if \(S \subset {\mathbb N}\) such that \(1 \in S\) and \(n + 1 \in S\) whenever \(n \in S\text{,}\) then \(S = {\mathbb N}\text{.}\)
15.
View Source for exercise
<exercise number="15">
<statement>
<p>For each of the following pairs of numbers <m>a</m> and <m>b</m>, calculate <m>\gcd(a,b)</m> and find integers <m>r</m> and <m>s</m> such that <m>\gcd(a,b) = ra + sb</m>.</p>
<ol cols="2">
<li><p>14 and 39</p></li>
<li><p>234 and 165</p></li>
<li><p>1739 and 9923</p></li>
<li><p>471 and 562</p></li>
<li><p>23,771 and 19,945</p></li>
<li><p><m>-4357</m> and 3754</p></li>
</ol>
</statement>
</exercise>
For each of the following pairs of numbers \(a\) and \(b\text{,}\) calculate \(\gcd(a,b)\) and find integers \(r\) and \(s\) such that \(\gcd(a,b) = ra + sb\text{.}\)
-
14 and 39
-
234 and 165
-
1739 and 9923
-
471 and 562
-
23,771 and 19,945
-
\(-4357\) and 3754
16.
View Source for exercise
<exercise number="16" xml:id="exercise-integers-gcd-1">
<statement>
<p>Let <m>a</m> and <m>b</m> be nonzero integers. If there exist integers <m>r</m> and <m>s</m> such that <m>ar + bs =1</m>, show that <m>a</m> and <m>b</m> are relatively prime.</p>
</statement>
</exercise>
Let \(a\) and \(b\) be nonzero integers. If there exist integers \(r\) and \(s\) such that \(ar + bs =1\text{,}\) show that \(a\) and \(b\) are relatively prime.
17. Fibonacci Numbers.
View Source for exercise
<exercise number="17">
<title>Fibonacci Numbers</title>
<statement>
<p>The Fibonacci numbers are
<md>1, 1, 2, 3, 5, 8, 13, 21, \ldots.</md>
We can define them inductively by <m>f_1 = 1</m>, <m>f_2 = 1</m>, and <m>f_{n + 2} = f_{n + 1} + f_n</m> for <m>n \in {\mathbb N}</m>.</p>
<ol>
<li xml:id="fn_ineq"><p>Prove that <m>f_n \lt 2^n</m>.</p></li>
<li xml:id="fn_prod"><p>Prove that <m>f_{n + 1} f_{n - 1} = f^2_n + (-1)^n</m>, <m>n \geq 2</m>.</p></li>
<li xml:id="fn_formula"><p>Prove that <m>f_n = [(1 + \sqrt{5}\, )^n - (1 - \sqrt{5}\, )^n]/ 2^n \sqrt{5}</m>.</p></li>
<li xml:id="fn_ratio"><p>Show that <m>\lim_{n \rightarrow \infty} f_n / f_{n + 1} = (\sqrt{5} - 1)/2</m>.</p></li>
<li xml:id="fn_coprime"><p>Prove that <m>f_n</m> and <m>f_{n + 1}</m> are relatively prime.</p></li>
</ol>
</statement>
<hint>
<p>For <xref ref="fn_ineq"/> and <xref ref="fn_prod"/> use mathematical induction.
<xref ref="fn_formula"/> Show that <m>f_1 = 1</m>, <m>f_2 = 1</m>, and <m>f_{n + 2} = f_{n + 1} + f_n</m>.
<xref ref="fn_ratio"/> Use part <xref ref="fn_formula"/>.
<xref ref="fn_coprime"/> Use part <xref ref="fn_prod"/> and <xref ref="exercise-integers-gcd-1"/>.</p>
</hint>
</exercise>
The Fibonacci numbers are
\begin{equation*}
1, 1, 2, 3, 5, 8, 13, 21, \ldots.
\end{equation*}
We can define them inductively by \(f_1 = 1\text{,}\) \(f_2 = 1\text{,}\) and \(f_{n + 2} = f_{n + 1} + f_n\) for \(n \in {\mathbb N}\text{.}\)
-
Prove that \(f_n \lt 2^n\text{.}\)
-
Prove that \(f_{n + 1} f_{n - 1} = f^2_n + (-1)^n\text{,}\) \(n \geq 2\text{.}\)
-
Prove that \(f_n = [(1 + \sqrt{5}\, )^n - (1 - \sqrt{5}\, )^n]/ 2^n \sqrt{5}\text{.}\)
-
Show that \(\lim_{n \rightarrow \infty} f_n / f_{n + 1} = (\sqrt{5} - 1)/2\text{.}\)
-
Prove that \(f_n\) and \(f_{n + 1}\) are relatively prime.
Hint.
View Source for hint
<hint>
<p>For <xref ref="fn_ineq"/> and <xref ref="fn_prod"/> use mathematical induction.
<xref ref="fn_formula"/> Show that <m>f_1 = 1</m>, <m>f_2 = 1</m>, and <m>f_{n + 2} = f_{n + 1} + f_n</m>.
<xref ref="fn_ratio"/> Use part <xref ref="fn_formula"/>.
<xref ref="fn_coprime"/> Use part <xref ref="fn_prod"/> and <xref ref="exercise-integers-gcd-1"/>.</p>
</hint>
For Item 2.4.17.a and Item 2.4.17.b use mathematical induction. Item 2.4.17.c Show that \(f_1 = 1\text{,}\) \(f_2 = 1\text{,}\) and \(f_{n + 2} = f_{n + 1} + f_n\text{.}\) Item 2.4.17.d Use part Item 2.4.17.c. Item 2.4.17.e Use part Item 2.4.17.b and Exercise 2.4.16.
18.
View Source for exercise
<exercise number="18">
<statement>
<p>Let <m>a</m> and <m>b</m> be integers such that <m>\gcd(a,b) = 1</m>. Let <m>r</m> and <m>s</m> be integers such that <m>ar + bs =1</m>. Prove that
<md>\gcd(a,s) = \gcd(r,b) = \gcd(r,s) = 1.</md></p>
</statement>
</exercise>
Let \(a\) and \(b\) be integers such that \(\gcd(a,b) = 1\text{.}\) Let \(r\) and \(s\) be integers such that \(ar + bs =1\text{.}\) Prove that
\begin{equation*}
\gcd(a,s) = \gcd(r,b) = \gcd(r,s) = 1.
\end{equation*}
19.
View Source for exercise
<exercise number="19">
<statement>
<p>Let <m>x, y \in {\mathbb N}</m> be relatively prime. If <m>xy</m> is a perfect square, prove that <m>x</m> and <m>y</m> must both be perfect squares.</p>
</statement>
<hint>
<p>Use the Fundamental Theorem of Arithmetic.</p>
</hint>
</exercise>
Let \(x, y \in {\mathbb N}\) be relatively prime. If \(xy\) is a perfect square, prove that \(x\) and \(y\) must both be perfect squares.
Hint.
View Source for hint
<hint>
<p>Use the Fundamental Theorem of Arithmetic.</p>
</hint>
Use the Fundamental Theorem of Arithmetic.
20.
View Source for exercise
<exercise number="20">
<statement>
<p>Using the division algorithm, show that every perfect square is of the form <m>4k</m> or <m>4k + 1</m> for some nonnegative integer <m>k</m>.</p>
</statement>
</exercise>
Using the division algorithm, show that every perfect square is of the form \(4k\) or \(4k + 1\) for some nonnegative integer \(k\text{.}\)
21.
View Source for exercise
<exercise number="21">
<statement>
<p>Suppose that <m>a, b, r, s</m> are pairwise relatively prime and that<md>
<mrow>a^2 + b^2 & = r^2</mrow>
<mrow>a^2 - b^2 & = s^2.</mrow>
</md></p>
<p>Prove that <m>a</m>, <m>r</m>, and <m>s</m> are odd and <m>b</m> is even.</p>
</statement>
</exercise>
Suppose that \(a, b, r, s\) are pairwise relatively prime and that
\begin{align*}
a^2 + b^2 & = r^2\\
a^2 - b^2 & = s^2.
\end{align*}
Prove that \(a\text{,}\) \(r\text{,}\) and \(s\) are odd and \(b\) is even.
22.
View Source for exercise
<exercise number="22">
<statement>
<p>Let <m>n \in {\mathbb N}</m>. Use the division algorithm to prove that every integer is congruent mod <m>n</m> to precisely one of the integers <m>0, 1, \ldots, n-1</m>. Conclude that if <m>r</m> is an integer, then there is exactly one <m>s</m> in <m>{\mathbb Z}</m> such that <m>0 \leq s \lt n</m> and <m>[r] = [s]</m>. Hence, the integers are indeed partitioned by congruence mod <m>n</m>.</p>
</statement>
</exercise>
Let \(n \in {\mathbb N}\text{.}\) Use the division algorithm to prove that every integer is congruent mod \(n\) to precisely one of the integers \(0, 1, \ldots, n-1\text{.}\) Conclude that if \(r\) is an integer, then there is exactly one \(s\) in \({\mathbb Z}\) such that \(0 \leq s \lt n\) and \([r] = [s]\text{.}\) Hence, the integers are indeed partitioned by congruence mod \(n\text{.}\)
23.
View Source for exercise
<exercise number="23">
<statement>
<p>Define the <term>least common multiple</term> of two nonzero integers <m>a</m> and <m>b</m>, denoted by <m>\lcm(a,b)</m>, to be the nonnegative integer <m>m</m> such that both <m>a</m> and <m>b</m> divide <m>m</m>, and if <m>a</m> and <m>b</m> divide any other integer <m>n</m>, then <m>m</m> also divides <m>n</m>. <notation><usage><m>\lcm(m,n)</m></usage><description>the least common multiple of <m>m</m> and <m>n</m></description></notation> Prove that any two integers <m>a</m> and <m>b</m> have a unique least common multiple.</p>
</statement>
<hint>
<p>Let <m>S = \{s \in {\mathbb N} : a \mid s</m>, <m>b \mid s \}</m>. Then <m>S \neq \emptyset</m>, since <m>|ab| \in S</m>. By the Principle of Well-Ordering, <m>S</m> contains a least element <m>m</m>. To show uniqueness, suppose that <m>a \mid n</m> and <m>b \mid n</m> for some <m>n \in {\mathbb N}</m>. By the division algorithm, there exist unique integers <m>q</m> and <m>r</m> such that <m>n = mq + r</m>, where <m>0 \leq r \lt m</m>. Since <m>a</m> and <m>b</m> divide both <m>m</m>, and <m>n</m>, it must be the case that <m>a</m> and <m>b</m> both divide <m>r</m>. Thus, <m>r = 0</m> by the minimality of <m>m</m>. Therefore, <m>m \mid n</m>.</p>
</hint>
</exercise>
Define the least common multiple of two nonzero integers \(a\) and \(b\text{,}\) denoted by \(\lcm(a,b)\text{,}\) to be the nonnegative integer \(m\) such that both \(a\) and \(b\) divide \(m\text{,}\) and if \(a\) and \(b\) divide any other integer \(n\text{,}\) then \(m\) also divides \(n\text{.}\) Prove that any two integers \(a\) and \(b\) have a unique least common multiple.
Hint.
View Source for hint
<hint>
<p>Let <m>S = \{s \in {\mathbb N} : a \mid s</m>, <m>b \mid s \}</m>. Then <m>S \neq \emptyset</m>, since <m>|ab| \in S</m>. By the Principle of Well-Ordering, <m>S</m> contains a least element <m>m</m>. To show uniqueness, suppose that <m>a \mid n</m> and <m>b \mid n</m> for some <m>n \in {\mathbb N}</m>. By the division algorithm, there exist unique integers <m>q</m> and <m>r</m> such that <m>n = mq + r</m>, where <m>0 \leq r \lt m</m>. Since <m>a</m> and <m>b</m> divide both <m>m</m>, and <m>n</m>, it must be the case that <m>a</m> and <m>b</m> both divide <m>r</m>. Thus, <m>r = 0</m> by the minimality of <m>m</m>. Therefore, <m>m \mid n</m>.</p>
</hint>
Let \(S = \{s \in {\mathbb N} : a \mid s\text{,}\) \(b \mid s \}\text{.}\) Then \(S \neq \emptyset\text{,}\) since \(|ab| \in S\text{.}\) By the Principle of Well-Ordering, \(S\) contains a least element \(m\text{.}\) To show uniqueness, suppose that \(a \mid n\) and \(b \mid n\) for some \(n \in {\mathbb N}\text{.}\) By the division algorithm, there exist unique integers \(q\) and \(r\) such that \(n = mq + r\text{,}\) where \(0 \leq r \lt m\text{.}\) Since \(a\) and \(b\) divide both \(m\text{,}\) and \(n\text{,}\) it must be the case that \(a\) and \(b\) both divide \(r\text{.}\) Thus, \(r = 0\) by the minimality of \(m\text{.}\) Therefore, \(m \mid n\text{.}\)
24.
View Source for exercise
<exercise number="24" xml:id="exercise-integers-lcm-gcd">
<statement>
<p>If <m>d= \gcd(a, b)</m> and <m>m = \lcm(a, b)</m>, prove that <m>dm = |ab|</m>.</p>
</statement>
</exercise>
If \(d= \gcd(a, b)\) and \(m = \lcm(a, b)\text{,}\) prove that \(dm = |ab|\text{.}\)
25.
View Source for exercise
<exercise number="25">
<statement>
<p>Show that <m>\lcm(a,b) = ab</m> if and only if <m>\gcd(a,b) = 1</m>.</p>
</statement>
</exercise>
Show that \(\lcm(a,b) = ab\) if and only if \(\gcd(a,b) = 1\text{.}\)
26.
View Source for exercise
<exercise number="26">
<statement>
<p>Prove that <m>\gcd(a,c) = \gcd(b,c) =1</m> if and only if <m>\gcd(ab,c) = 1</m> for integers <m>a</m>, <m>b</m>, and <m>c</m>.</p>
</statement>
</exercise>
Prove that \(\gcd(a,c) = \gcd(b,c) =1\) if and only if \(\gcd(ab,c) = 1\) for integers \(a\text{,}\) \(b\text{,}\) and \(c\text{.}\)
27.
View Source for exercise
<exercise number="27">
<statement>
<p>Let <m>a, b, c \in {\mathbb Z}</m>. Prove that if <m>\gcd(a,b) = 1</m> and <m>a \mid bc</m>, then <m>a \mid c</m>. </p>
</statement>
<hint>
<p>Since <m>\gcd(a,b) = 1</m>, there exist integers <m>r</m> and <m>s</m> such that <m>ar + bs = 1</m>. Thus, <m>acr + bcs = c</m>. Since <m>a</m> divides both <m>bc</m> and itself, <m>a</m> must divide <m>c</m>.</p>
</hint>
</exercise>
Let \(a, b, c \in {\mathbb Z}\text{.}\) Prove that if \(\gcd(a,b) = 1\) and \(a \mid bc\text{,}\) then \(a \mid c\text{.}\)
Hint.
View Source for hint
<hint>
<p>Since <m>\gcd(a,b) = 1</m>, there exist integers <m>r</m> and <m>s</m> such that <m>ar + bs = 1</m>. Thus, <m>acr + bcs = c</m>. Since <m>a</m> divides both <m>bc</m> and itself, <m>a</m> must divide <m>c</m>.</p>
</hint>
Since \(\gcd(a,b) = 1\text{,}\) there exist integers \(r\) and \(s\) such that \(ar + bs = 1\text{.}\) Thus, \(acr + bcs = c\text{.}\) Since \(a\) divides both \(bc\) and itself, \(a\) must divide \(c\text{.}\)
28.
View Source for exercise
<exercise number="28">
<statement>
<p>Let <m>p \geq 2</m>. Prove that if <m>2^p - 1</m> is prime, then <m>p</m> must also be prime.</p>
</statement>
</exercise>
Let \(p \geq 2\text{.}\) Prove that if \(2^p - 1\) is prime, then \(p\) must also be prime.
29.
View Source for exercise
<exercise number="29">
<statement>
<p>Prove that there are an infinite number of primes of the form <m>6n + 5</m>. </p>
</statement>
<hint>
<p>Every prime must be of the form 2, 3, <m>6n + 1</m>, or <m>6n + 5</m>. Suppose there are only finitely many primes of the form <m>6k + 5</m>.</p>
</hint>
</exercise>
Prove that there are an infinite number of primes of the form \(6n + 5\text{.}\)
Hint.
View Source for hint
<hint>
<p>Every prime must be of the form 2, 3, <m>6n + 1</m>, or <m>6n + 5</m>. Suppose there are only finitely many primes of the form <m>6k + 5</m>.</p>
</hint>
Every prime must be of the form 2, 3, \(6n + 1\text{,}\) or \(6n + 5\text{.}\) Suppose there are only finitely many primes of the form \(6k + 5\text{.}\)
30.
View Source for exercise
<exercise number="30">
<statement>
<p>Prove that there are an infinite number of primes of the form <m>4n - 1</m>.</p>
</statement>
</exercise>
Prove that there are an infinite number of primes of the form \(4n - 1\text{.}\)
31.
View Source for exercise
<exercise number="31">
<statement>
<p>Using the fact that 2 is prime, show that there do not exist integers <m>p</m> and <m>q</m> such that <m>p^2 = 2 q^2</m>. Demonstrate that therefore <m>\sqrt{2}</m> cannot be a rational number.</p>
</statement>
</exercise>
Using the fact that 2 is prime, show that there do not exist integers \(p\) and \(q\) such that \(p^2 = 2 q^2\text{.}\) Demonstrate that therefore \(\sqrt{2}\) cannot be a rational number.
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