diff --git a/chapter_5/exercises.md b/chapter_5/exercises.md index 1cda74a..79f18df 100644 --- a/chapter_5/exercises.md +++ b/chapter_5/exercises.md @@ -10457,27 +10457,385 @@ verify that the formula of part (a) is correct. 43. $a_k = \dfrac{a_{k - 1}}{2a_{k - 1} - 1}$, for each integer $k \geq 1$ $a_0 = 2$. +a. + +$$ a_0 = 2 $$ + +$$ a_1 = \frac{a_0}{2a_0 - 1} = \frac{2}{2(2) - 1} = \frac{2}{3} $$ + +$$ a_2 = \frac{a_1}{2a_1 - 1} = \frac{\dfrac{2}{3}}{2\left(\dfrac{2}{3}\right) - 1} = \frac{2}{4 - 3} = 2 $$ + +$$ a_3 = \frac{a_2}{2a_2 - 1} = \frac{2}{2(2) - 1} = \frac{2}{3} $$ + +Guess: + +$$ +a_n = +\begin{cases} +2 & \text{if } n \text{ is even} \\ +\dfrac{2}{3} & n \text{ is odd} +\end{cases} +$$ + +b. + +Let $a_0, a_1, a_2, \dots$ be the sequence defined recursively by $a_0 = 2$ and +$a_k = \dfrac{a_{k - 1}}{2a_{k - 1} - 1}$ for each integer $k \geq 1$. + +Let the property $P(n)$ be the equation: + +$$ +a_n = +\begin{cases} +2 & \text{if } n \text{ is even} \\ +\dfrac{2}{3} & \text{if } n \text{ is odd} +\end{cases} +$$ + +**Proof by strong mathematical induction:** + +_Basis Step:_ + +Prove $P(0)$ and $P(1)$, that is: + +$$ +a_0 = +\begin{cases} +2 & \text{if } 0 \text{ is even} \\ +\dfrac{2}{3} & \text{if } 0 \text{ is odd} +\end{cases} +$$ + +and: + +$$ +a_1 = +\begin{cases} +2 & \text{if } 1 \text{ is even} \\ +\dfrac{2}{3} & \text{if } 1 \text{ is odd} +\end{cases} +$$ + +$P(0)$ is true since the piecewise function tells us that $a_0 = 2$ since $0$ is +even and this matches the given value of $a_0 = 2$. + +$P(1)$ is true given the evaluation of $a_1$ in part (a). + +Therefore both $P(0)$ and $P(1)$ are true. + +_Inductive Step:_ + +Let $k$ be any integer such that $k \geq 0$. Let $i$ be some integer such that +$0 \leq i \leq k$. + +Suppose $P(i)$, that is: + +$$ +a_i = +\begin{cases} +2 & \text{if } i \text{ is even} \\ +\dfrac{2}{3} & \text{if } i \text{ is odd} +\end{cases} +$$ + +This is the inductive hypothesis. + +Prove $P(k + 1)$, that is: + +$$ +a_{k + 1} = +\begin{cases} +2 & \text{if } k + 1 \text{ is even} \\ +\dfrac{2}{3} & \text{if } k + 1 \text{ is odd} +\end{cases} +$$ + +By the definition of the sequence: + +$$ a_k = \dfrac{a_{k - 1}}{2a_{k - 1} - 1} $$ + +It follows that: + +$$ a_{k + 1} = \dfrac{a_k}{2a_k - 1} $$ + +By the inductive hypothesis: + +$$ +a_{k + 1} = +\begin{cases} +\dfrac{2}{2(2) - 1} & \text{if } k \text{ is even} \\ +\dfrac{\dfrac{2}{3}}{2\left(\dfrac{2}{3}\right) - 1} & \text{if } k \text{ is odd} +\end{cases} +$$ + +$$ += \\ +\begin{cases} +\dfrac{2}{3} & \text{if } k \text{ is even} \\ +2 & \text{if } k \text{ is odd} +\end{cases} +$$ + +It follows that: + +$$ += \\ +\begin{cases} +\dfrac{2}{3} & \text{if } k + 1 \text{ is odd} \\ +2 & \text{if } k + 1 \text{ is even} +\end{cases} +$$ + +This is what was to be shown. Therefore $P(k + 1)$ is true. + +Q.E.D. + 44. $b_k = \dfrac{2}{b_{k - 1}}$, for each integer $k \geq 2$ $b_1 = 1$. +a. + +$$ b_1 = 1 $$ + +$$ b_2 = \frac{2}{b_1} = \frac{2}{1} = 2 $$ + +$$ b_3 = \frac{2}{b_2} = \frac{2}{2} = 1 $$ + +Guess: + +$$ +b_n = +\begin{cases} +1 & \text{if } n \text{ is odd} \\ +2 & \text{if } n \text{ is even} +\end{cases} +$$ + +b. + +Let $b_1, b_2, b_3, \dots$ be the sequence defined recursively by $b_1 = 1$ and +$b_k = \dfrac{2}{b_{k - 1}}$ for each integer $k \geq 2$. + +**Proof by strong mathematical induction:** + +Let the property $P(n)$ be the equation: + +$$ +b_n = +\begin{cases} +1 & \text{if } n \text{ is odd} \\ +2 & \text{if } n \text{ is even} +\end{cases} +$$ + +_Basis Step:_ + +Prove $P(1)$ and $P(2)$. + +Both $P(1)$ and $P(2)$ are proven true in part (a). + +_Inductive Step:_ + +Let $k$ be any integer such that $k \geq 2$, and let $i$ be some integer such +that $1 \leq i \leq k$. + +Suppose $P(i)$, that is: + +$$ +b_i = +\begin{cases} +1 & \text{if } i \text{ is odd} \\ +2 & \text{if } i \text{ is even} +\end{cases} +$$ + +This is the inductive hypothesis. + +Prove $P(k + 1)$, that is: + +$$ +b_{k + 1} = +\begin{cases} +1 & \text{if } k + 1 \text{ is odd} \\ +2 & \text{if } k + 1 \text{ is even} +\end{cases} +$$ + +By the definition of the given sequence: + +$$ b_k = \dfrac{2}{b_{k - 1}} $$ + +It follows that: + +$$ b_{k + 1} = \dfrac{2}{b_k} $$ + +By the inductive hypothesis: + +$$ +b_{k + 1} = +\begin{cases} +\dfrac{2}{1} & \text{if } k \text{ is odd} \\ +\dfrac{2}{2} & \text{if } k \text{ is even} +\end{cases} +$$ + +$$ += \\ +\begin{cases} +2 & \text{if } k \text{ is odd} \\ +1 & \text{if } k \text{ is even} +\end{cases} +$$ + +$$ += \\ +\begin{cases} +2 & \text{if } k + 1 \text{ is even} \\ +1 & \text{if } k + 1 \text{ is odd} +\end{cases} +$$ + +This is what was to be shown. Therefore $P(k + 1)$ is true. + +Q.E.D. + 45. $v_k = v_{\lfloor \dfrac{k}{2} \rfloor} + v_{\lfloor \dfrac{(k + 1)}{2}\rfloor} + 2$, for each integer $k \geq 2$ $v_1 = 1$. +a. + +$$ v_1 = 1 $$ + +$$ v_2 = v_{\lfloor \dfrac{2}{2} \rfloor} + v_{\lfloor \dfrac{(2 + 1)}{2} \rfloor} + 2 $$ + +$$ = v_1 + v_1 + 2 = 1 + 1 + 2 $$ + +$$ v_3 = v_{\lfloor \dfrac{3}{2} \rfloor} + v_{\lfloor \dfrac{(3 + 1)}{2} \rfloor} + 2 $$ + +$$ = v_1 + v_2 + 2 = 1 + (1 + 1 + 2) + 2 $$ + +$$ = 3 + 2 \cdot 2 $$ + +$$ v_4 = v_{\lfloor \dfrac{4}{2} \rfloor} + v_{\lfloor \dfrac{(4 + 1)}{2} \rfloor} + 2 $$ + +$$ = v_2 + v_2 + 2 = (1 + 1 + 2) + (1 + 1 + 2) + 2 $$ + +$$ = 4 + 3 \cdot 2 $$ + +$$ v_5 = v_{\lfloor \dfrac{5}{2} \rfloor} + v_{\lfloor \dfrac{(5 + 1)}{2} \rfloor} + 2 $$ + +$$ = v_2 + v_3 + 2 = (1 + 1 + 2) + (3 + 2 \cdot 2) + 2 $$ + +$$ = 5 + 4 \cdot 2 $$ + +Guess: + +$$ v_n = n + 2(n - 1) = n + 2n - 2 $$ + +$$ = 3n - 2 $$ + +b. + +Let $v_1, v_2, v_3, \dots$ be the sequence defined recursively by $v_1 = 1$ and +$v_k = v_{\lfloor \frac{k}{2} \rfloor} + v_{\lfloor \frac{(k + 1)}{2} \rfloor} + 2$ +for each integer $k \geq 2$. + +**Proof by strong mathematical induction:** + +Let the property $P(n)$ be the equation $v_n = 3n - 2$. + +_Basis Step:_ + +Prove $P(1)$, that is: + +$$ v_1 = 3(1) - 2 $$ + +$$ = 3 - 2 $$ + +$$ = 1 $$ + +This equality matches the given value of $v_1 = 1$. Therefore $P(1)$ is true. + +_Inductive Step:_ + +Let $k$ be any integer such that $k \geq 2$, and let $i$ be some integer such +that $1 \leq i \leq k$. + +Suppose $P(i)$, that is: + +$$ v_i = 3i - 2 $$ + +This is the inductive hypothesis. + +Prove $P(k + 1)$, that is: + +$$ v_{k + 1} = 3(k + 1) - 2 $$ + +Alternatively: + +$$ v_{k + 1} = 3k + 1 $$ + +By the definition of the given sequence: + +$$ v_k = v_{\lfloor \frac{k}{2} \rfloor} + v_{\lfloor \frac{(k + 1)}{2} \rfloor} + 2 $$ + +It follows that: + +$$ v_{k + 1} = v_{\lfloor \frac{(k + 1)}{2} \rfloor} + v_{\lfloor \frac{(k + 2)}{2} \rfloor} + 2 $$ + +By the inductive hypothesis: + +$$ = \left(3\lfloor \frac{k + 1}{2} \rfloor - 2\right) + \left(3\lfloor \frac{k + 2}{2}\rfloor - 2\right) + 2 $$ + +$$ = 3\left(\lfloor \frac{k + 1}{2} \rfloor + \lfloor \frac{k + 2}{2} \rfloor \right) - 2 $$ + +$$ += \\ +\begin{cases} +3\left(\frac{k}{2} + \frac{k + 2}{2}\right) - 2 & \text{if } k \text{ is even} \\ +3\left(\frac{k + 1}{2} + \frac{k + 1}{2}\right) - 2 & \text{if } k \text{ is odd} +\end{cases} +$$ + +$$ = 3\left(\frac{2k + 2}{2}\right) - 2 $$ + +$$ = 3(k + 1) - 2 $$ + +$$ = 3k + 3 - 2 $$ + +$$ = 3k + 1 $$ + +This is what was to be shown. Therefore $P(k + 1)$ is true. + +Q.E.D. + 46. $s_k = 2s_{k - 2}$, for each integer $k \geq 2$ $s_0 = 1$, $s_1 = 2$. +Omitted. + 47. $t_k = k - t_{k - 1}$, for each integer $k \geq 1$ $t_0 = 0$. +Omitted. + 48. $w_k = w_{k - 2} + k$, for each integer $k \geq 3$ $w_1 = 1$, $w_2 = 2$. +Omitted. + 49. $u_k = u_{k - 2} \cdot u_{k - 1}$, for each integer $k \geq 2$ $u_0 = u_1 = 2$ +Omitted. + In 50 and 51 determine whether the given recursively defined sequence satisfies the explicit formula $a_n = (n - 1)^2$, for every integer $n \geq 1$. 50. $a_k = 2a_{k - 1} + k - 1$, for each integer $k \geq 2$ $a_1 = 0$. +Omitted. + 51. $a_k = 4a_{k - 1} - k + 3$, for each integer $k \geq 2$ $a_1 = 0$. +Omitted. + 52. A single line divides a plane into two regions. Two lines (by crossing) can divide a plane into four regions; three lines can divide it into seven regions (see the figure). Let $P_n$ be the maximum number of regions into @@ -10488,13 +10846,19 @@ the explicit formula $a_n = (n - 1)^2$, for every integer $n \geq 1$. a. Derive a recurrence relation for $P_k$ in terms of $P_{k - 1}$, for each integer $k \geq 2$. +Omitted. + b. Use iteration to guess an explicit formula for $P_n$. +Omitted. + 53. Compute $\left[\begin{array}{cc} 1 & 1 \\ 1 & 0 \\ \end{array}\right]^n$ for small values of $n$ (up to about 5 or 6). Conjecture explicit formulas for the entries in this matrix, and prove your conjecture using mathematical induction. +Omitted. + 54. In economics the behavior of an economy from one period to another is often modeled by recurrence relations. Let $Y_k$ be the income in period $k$ and $C_k$ be the consumption in period $k$. In one economic model, income in any @@ -10516,9 +10880,13 @@ $Y_k = E + c + mY_{k - 1}$. a. Use iteration on the above recurrence relation to obtain +Omitted. + $$ Y_n = (E + c)\left(\frac{m^n - 1}{m - 1}\right) + m^nY_0 $$ for every integer $n \geq 1$. b. (For students who have studied calculus) Show that if $0 < m < 1$, then $\lim\limits_{m \to \infty}Y_n = \dfrac{E + c}{1 - m}$. + +Omitted.