Show that $f(a)=f(a-1).$
$begingroup$
Suppose that $f :[0,2]rightarrow mathbb{R}$ is a continuous function with $f(0)=f(2)$. Show that there is a real number $ain[1,2]$ with $f(a)=f(a-1)$.
For the answer I tried to apply the mean value theorem to the function $g(x)=int_0^2f(t)dt$ in the interval $[0,2]$ but it didn't lead any where.
calculus analysis
$endgroup$
add a comment |
$begingroup$
Suppose that $f :[0,2]rightarrow mathbb{R}$ is a continuous function with $f(0)=f(2)$. Show that there is a real number $ain[1,2]$ with $f(a)=f(a-1)$.
For the answer I tried to apply the mean value theorem to the function $g(x)=int_0^2f(t)dt$ in the interval $[0,2]$ but it didn't lead any where.
calculus analysis
$endgroup$
7
$begingroup$
You want $f(a)=f(a-1)$. So the most immediately obvious IVT function to try would be $f(x)-f(x-1)$.
$endgroup$
– Arthur
Dec 18 '18 at 21:22
3
$begingroup$
Define $g(x)=f(x)-f(x-1)$, Then consider $g$ at $x=1$ and $x=2$ $g(1)=f(1)$ $g(2)=-f(1)$ so they have opposite sign... (well... if they don't have opposite sign then it's even easier... )
$endgroup$
– Mason
Dec 18 '18 at 21:24
$begingroup$
.Got it. Thank you!
$endgroup$
– Charith Jeewantha
Dec 18 '18 at 21:33
add a comment |
$begingroup$
Suppose that $f :[0,2]rightarrow mathbb{R}$ is a continuous function with $f(0)=f(2)$. Show that there is a real number $ain[1,2]$ with $f(a)=f(a-1)$.
For the answer I tried to apply the mean value theorem to the function $g(x)=int_0^2f(t)dt$ in the interval $[0,2]$ but it didn't lead any where.
calculus analysis
$endgroup$
Suppose that $f :[0,2]rightarrow mathbb{R}$ is a continuous function with $f(0)=f(2)$. Show that there is a real number $ain[1,2]$ with $f(a)=f(a-1)$.
For the answer I tried to apply the mean value theorem to the function $g(x)=int_0^2f(t)dt$ in the interval $[0,2]$ but it didn't lead any where.
calculus analysis
calculus analysis
edited Dec 18 '18 at 21:23
user376343
3,8733829
3,8733829
asked Dec 18 '18 at 21:19
Charith JeewanthaCharith Jeewantha
898
898
7
$begingroup$
You want $f(a)=f(a-1)$. So the most immediately obvious IVT function to try would be $f(x)-f(x-1)$.
$endgroup$
– Arthur
Dec 18 '18 at 21:22
3
$begingroup$
Define $g(x)=f(x)-f(x-1)$, Then consider $g$ at $x=1$ and $x=2$ $g(1)=f(1)$ $g(2)=-f(1)$ so they have opposite sign... (well... if they don't have opposite sign then it's even easier... )
$endgroup$
– Mason
Dec 18 '18 at 21:24
$begingroup$
.Got it. Thank you!
$endgroup$
– Charith Jeewantha
Dec 18 '18 at 21:33
add a comment |
7
$begingroup$
You want $f(a)=f(a-1)$. So the most immediately obvious IVT function to try would be $f(x)-f(x-1)$.
$endgroup$
– Arthur
Dec 18 '18 at 21:22
3
$begingroup$
Define $g(x)=f(x)-f(x-1)$, Then consider $g$ at $x=1$ and $x=2$ $g(1)=f(1)$ $g(2)=-f(1)$ so they have opposite sign... (well... if they don't have opposite sign then it's even easier... )
$endgroup$
– Mason
Dec 18 '18 at 21:24
$begingroup$
.Got it. Thank you!
$endgroup$
– Charith Jeewantha
Dec 18 '18 at 21:33
7
7
$begingroup$
You want $f(a)=f(a-1)$. So the most immediately obvious IVT function to try would be $f(x)-f(x-1)$.
$endgroup$
– Arthur
Dec 18 '18 at 21:22
$begingroup$
You want $f(a)=f(a-1)$. So the most immediately obvious IVT function to try would be $f(x)-f(x-1)$.
$endgroup$
– Arthur
Dec 18 '18 at 21:22
3
3
$begingroup$
Define $g(x)=f(x)-f(x-1)$, Then consider $g$ at $x=1$ and $x=2$ $g(1)=f(1)$ $g(2)=-f(1)$ so they have opposite sign... (well... if they don't have opposite sign then it's even easier... )
$endgroup$
– Mason
Dec 18 '18 at 21:24
$begingroup$
Define $g(x)=f(x)-f(x-1)$, Then consider $g$ at $x=1$ and $x=2$ $g(1)=f(1)$ $g(2)=-f(1)$ so they have opposite sign... (well... if they don't have opposite sign then it's even easier... )
$endgroup$
– Mason
Dec 18 '18 at 21:24
$begingroup$
.Got it. Thank you!
$endgroup$
– Charith Jeewantha
Dec 18 '18 at 21:33
$begingroup$
.Got it. Thank you!
$endgroup$
– Charith Jeewantha
Dec 18 '18 at 21:33
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
Define a function $gcolon [1,2] to mathbb{R}$ by $g(x) = f(x) - f(x-1)$, so that you're looking for an $a$ such that $g(a) = 0$. Check that $g$ is continuous and apply the Intermediate Value Theorem.
You need to know something about $f(1)$ to do this properly, but you can consider separately the three cases where $f(1)$ is greater than, less than, or (the trivial case) equal to $f(0)$ and $f(2)$.
ETA: While I was writing this, two comments appeared suggesting the same approach.
$endgroup$
add a comment |
$begingroup$
Let $g(x) = f(x) - f(x-1)$
$g(2) = f(2) - f(1) = f(0) - f(1) = -g(1)$
if $g(1) = 0$ you are done.
Otherwise $g(x)$ is a continuous function that is negative at one endpoint and positive at the other.
By the IVT there must be an $ain [1,2]$ such that $g(a) = 0 $
$endgroup$
$begingroup$
"if g(x)=0 you are done." should be g(1)=0
$endgroup$
– Toby Bartels
Dec 18 '18 at 21:29
$begingroup$
@TobyBartels thanks.
$endgroup$
– Doug M
Dec 18 '18 at 21:29
add a comment |
$begingroup$
Elementary Approach using Bolzano's Theorem and not IVT :
Let $g(x) = f(x) - f(x-1)$. This function is continuous in $[0,2]$ as an operation of continuous functions.
Now, $g(1) = f(1)- f(0)$ and $g(2) = f(2)-f(1)$.
$$g(1)cdot g(2) =f(1)f(2)-f(1)^2 - f(2)f(0)+ f(0)f(1)=2f(0)f(1) - f(1)^2-f(0)^2 $$
$$implies$$
$$g(1)cdot g(2) = -big[f(1)-f(0)big]^2 leq 0 $$
Thus, there exists $a in [0,2]$ such that $g(a) =0$ by Bolzano's Theorem.
Functional Analysis (sorry for this) :
Essentialy, we want to show that the equation
$$f(x) - f(x-1) =0$$
has a solution.
Let $T$ be a linear operator $T:C[0,2] to C[0,2]$ such that $Tf(x) = -f(x-1)$. One can easily see that $T$ is a bounded linear operator $in Bbig(C[0,2]big)$. Also, one can take $0$ to be a function defined in $C[0,2]$. Then, the equation is re-written as :
$$f + Tf = 0$$
Note that $C[0,2]$ is a Banach Space and since $T in Bbig(C[0,2]big)$ then there exists a unique solution in $C[0,2]$, meaning that there exists such function $f(x) in C[0,2]$ which of course implies that the equation has a solution for $x in [0,2]$.
$endgroup$
add a comment |
Your Answer
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3 Answers
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3 Answers
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$begingroup$
Define a function $gcolon [1,2] to mathbb{R}$ by $g(x) = f(x) - f(x-1)$, so that you're looking for an $a$ such that $g(a) = 0$. Check that $g$ is continuous and apply the Intermediate Value Theorem.
You need to know something about $f(1)$ to do this properly, but you can consider separately the three cases where $f(1)$ is greater than, less than, or (the trivial case) equal to $f(0)$ and $f(2)$.
ETA: While I was writing this, two comments appeared suggesting the same approach.
$endgroup$
add a comment |
$begingroup$
Define a function $gcolon [1,2] to mathbb{R}$ by $g(x) = f(x) - f(x-1)$, so that you're looking for an $a$ such that $g(a) = 0$. Check that $g$ is continuous and apply the Intermediate Value Theorem.
You need to know something about $f(1)$ to do this properly, but you can consider separately the three cases where $f(1)$ is greater than, less than, or (the trivial case) equal to $f(0)$ and $f(2)$.
ETA: While I was writing this, two comments appeared suggesting the same approach.
$endgroup$
add a comment |
$begingroup$
Define a function $gcolon [1,2] to mathbb{R}$ by $g(x) = f(x) - f(x-1)$, so that you're looking for an $a$ such that $g(a) = 0$. Check that $g$ is continuous and apply the Intermediate Value Theorem.
You need to know something about $f(1)$ to do this properly, but you can consider separately the three cases where $f(1)$ is greater than, less than, or (the trivial case) equal to $f(0)$ and $f(2)$.
ETA: While I was writing this, two comments appeared suggesting the same approach.
$endgroup$
Define a function $gcolon [1,2] to mathbb{R}$ by $g(x) = f(x) - f(x-1)$, so that you're looking for an $a$ such that $g(a) = 0$. Check that $g$ is continuous and apply the Intermediate Value Theorem.
You need to know something about $f(1)$ to do this properly, but you can consider separately the three cases where $f(1)$ is greater than, less than, or (the trivial case) equal to $f(0)$ and $f(2)$.
ETA: While I was writing this, two comments appeared suggesting the same approach.
answered Dec 18 '18 at 21:24
Toby BartelsToby Bartels
669515
669515
add a comment |
add a comment |
$begingroup$
Let $g(x) = f(x) - f(x-1)$
$g(2) = f(2) - f(1) = f(0) - f(1) = -g(1)$
if $g(1) = 0$ you are done.
Otherwise $g(x)$ is a continuous function that is negative at one endpoint and positive at the other.
By the IVT there must be an $ain [1,2]$ such that $g(a) = 0 $
$endgroup$
$begingroup$
"if g(x)=0 you are done." should be g(1)=0
$endgroup$
– Toby Bartels
Dec 18 '18 at 21:29
$begingroup$
@TobyBartels thanks.
$endgroup$
– Doug M
Dec 18 '18 at 21:29
add a comment |
$begingroup$
Let $g(x) = f(x) - f(x-1)$
$g(2) = f(2) - f(1) = f(0) - f(1) = -g(1)$
if $g(1) = 0$ you are done.
Otherwise $g(x)$ is a continuous function that is negative at one endpoint and positive at the other.
By the IVT there must be an $ain [1,2]$ such that $g(a) = 0 $
$endgroup$
$begingroup$
"if g(x)=0 you are done." should be g(1)=0
$endgroup$
– Toby Bartels
Dec 18 '18 at 21:29
$begingroup$
@TobyBartels thanks.
$endgroup$
– Doug M
Dec 18 '18 at 21:29
add a comment |
$begingroup$
Let $g(x) = f(x) - f(x-1)$
$g(2) = f(2) - f(1) = f(0) - f(1) = -g(1)$
if $g(1) = 0$ you are done.
Otherwise $g(x)$ is a continuous function that is negative at one endpoint and positive at the other.
By the IVT there must be an $ain [1,2]$ such that $g(a) = 0 $
$endgroup$
Let $g(x) = f(x) - f(x-1)$
$g(2) = f(2) - f(1) = f(0) - f(1) = -g(1)$
if $g(1) = 0$ you are done.
Otherwise $g(x)$ is a continuous function that is negative at one endpoint and positive at the other.
By the IVT there must be an $ain [1,2]$ such that $g(a) = 0 $
edited Dec 18 '18 at 21:29
answered Dec 18 '18 at 21:26
Doug MDoug M
45.3k31954
45.3k31954
$begingroup$
"if g(x)=0 you are done." should be g(1)=0
$endgroup$
– Toby Bartels
Dec 18 '18 at 21:29
$begingroup$
@TobyBartels thanks.
$endgroup$
– Doug M
Dec 18 '18 at 21:29
add a comment |
$begingroup$
"if g(x)=0 you are done." should be g(1)=0
$endgroup$
– Toby Bartels
Dec 18 '18 at 21:29
$begingroup$
@TobyBartels thanks.
$endgroup$
– Doug M
Dec 18 '18 at 21:29
$begingroup$
"if g(x)=0 you are done." should be g(1)=0
$endgroup$
– Toby Bartels
Dec 18 '18 at 21:29
$begingroup$
"if g(x)=0 you are done." should be g(1)=0
$endgroup$
– Toby Bartels
Dec 18 '18 at 21:29
$begingroup$
@TobyBartels thanks.
$endgroup$
– Doug M
Dec 18 '18 at 21:29
$begingroup$
@TobyBartels thanks.
$endgroup$
– Doug M
Dec 18 '18 at 21:29
add a comment |
$begingroup$
Elementary Approach using Bolzano's Theorem and not IVT :
Let $g(x) = f(x) - f(x-1)$. This function is continuous in $[0,2]$ as an operation of continuous functions.
Now, $g(1) = f(1)- f(0)$ and $g(2) = f(2)-f(1)$.
$$g(1)cdot g(2) =f(1)f(2)-f(1)^2 - f(2)f(0)+ f(0)f(1)=2f(0)f(1) - f(1)^2-f(0)^2 $$
$$implies$$
$$g(1)cdot g(2) = -big[f(1)-f(0)big]^2 leq 0 $$
Thus, there exists $a in [0,2]$ such that $g(a) =0$ by Bolzano's Theorem.
Functional Analysis (sorry for this) :
Essentialy, we want to show that the equation
$$f(x) - f(x-1) =0$$
has a solution.
Let $T$ be a linear operator $T:C[0,2] to C[0,2]$ such that $Tf(x) = -f(x-1)$. One can easily see that $T$ is a bounded linear operator $in Bbig(C[0,2]big)$. Also, one can take $0$ to be a function defined in $C[0,2]$. Then, the equation is re-written as :
$$f + Tf = 0$$
Note that $C[0,2]$ is a Banach Space and since $T in Bbig(C[0,2]big)$ then there exists a unique solution in $C[0,2]$, meaning that there exists such function $f(x) in C[0,2]$ which of course implies that the equation has a solution for $x in [0,2]$.
$endgroup$
add a comment |
$begingroup$
Elementary Approach using Bolzano's Theorem and not IVT :
Let $g(x) = f(x) - f(x-1)$. This function is continuous in $[0,2]$ as an operation of continuous functions.
Now, $g(1) = f(1)- f(0)$ and $g(2) = f(2)-f(1)$.
$$g(1)cdot g(2) =f(1)f(2)-f(1)^2 - f(2)f(0)+ f(0)f(1)=2f(0)f(1) - f(1)^2-f(0)^2 $$
$$implies$$
$$g(1)cdot g(2) = -big[f(1)-f(0)big]^2 leq 0 $$
Thus, there exists $a in [0,2]$ such that $g(a) =0$ by Bolzano's Theorem.
Functional Analysis (sorry for this) :
Essentialy, we want to show that the equation
$$f(x) - f(x-1) =0$$
has a solution.
Let $T$ be a linear operator $T:C[0,2] to C[0,2]$ such that $Tf(x) = -f(x-1)$. One can easily see that $T$ is a bounded linear operator $in Bbig(C[0,2]big)$. Also, one can take $0$ to be a function defined in $C[0,2]$. Then, the equation is re-written as :
$$f + Tf = 0$$
Note that $C[0,2]$ is a Banach Space and since $T in Bbig(C[0,2]big)$ then there exists a unique solution in $C[0,2]$, meaning that there exists such function $f(x) in C[0,2]$ which of course implies that the equation has a solution for $x in [0,2]$.
$endgroup$
add a comment |
$begingroup$
Elementary Approach using Bolzano's Theorem and not IVT :
Let $g(x) = f(x) - f(x-1)$. This function is continuous in $[0,2]$ as an operation of continuous functions.
Now, $g(1) = f(1)- f(0)$ and $g(2) = f(2)-f(1)$.
$$g(1)cdot g(2) =f(1)f(2)-f(1)^2 - f(2)f(0)+ f(0)f(1)=2f(0)f(1) - f(1)^2-f(0)^2 $$
$$implies$$
$$g(1)cdot g(2) = -big[f(1)-f(0)big]^2 leq 0 $$
Thus, there exists $a in [0,2]$ such that $g(a) =0$ by Bolzano's Theorem.
Functional Analysis (sorry for this) :
Essentialy, we want to show that the equation
$$f(x) - f(x-1) =0$$
has a solution.
Let $T$ be a linear operator $T:C[0,2] to C[0,2]$ such that $Tf(x) = -f(x-1)$. One can easily see that $T$ is a bounded linear operator $in Bbig(C[0,2]big)$. Also, one can take $0$ to be a function defined in $C[0,2]$. Then, the equation is re-written as :
$$f + Tf = 0$$
Note that $C[0,2]$ is a Banach Space and since $T in Bbig(C[0,2]big)$ then there exists a unique solution in $C[0,2]$, meaning that there exists such function $f(x) in C[0,2]$ which of course implies that the equation has a solution for $x in [0,2]$.
$endgroup$
Elementary Approach using Bolzano's Theorem and not IVT :
Let $g(x) = f(x) - f(x-1)$. This function is continuous in $[0,2]$ as an operation of continuous functions.
Now, $g(1) = f(1)- f(0)$ and $g(2) = f(2)-f(1)$.
$$g(1)cdot g(2) =f(1)f(2)-f(1)^2 - f(2)f(0)+ f(0)f(1)=2f(0)f(1) - f(1)^2-f(0)^2 $$
$$implies$$
$$g(1)cdot g(2) = -big[f(1)-f(0)big]^2 leq 0 $$
Thus, there exists $a in [0,2]$ such that $g(a) =0$ by Bolzano's Theorem.
Functional Analysis (sorry for this) :
Essentialy, we want to show that the equation
$$f(x) - f(x-1) =0$$
has a solution.
Let $T$ be a linear operator $T:C[0,2] to C[0,2]$ such that $Tf(x) = -f(x-1)$. One can easily see that $T$ is a bounded linear operator $in Bbig(C[0,2]big)$. Also, one can take $0$ to be a function defined in $C[0,2]$. Then, the equation is re-written as :
$$f + Tf = 0$$
Note that $C[0,2]$ is a Banach Space and since $T in Bbig(C[0,2]big)$ then there exists a unique solution in $C[0,2]$, meaning that there exists such function $f(x) in C[0,2]$ which of course implies that the equation has a solution for $x in [0,2]$.
edited Dec 18 '18 at 21:40
answered Dec 18 '18 at 21:34
RebellosRebellos
15k31249
15k31249
add a comment |
add a comment |
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$begingroup$
You want $f(a)=f(a-1)$. So the most immediately obvious IVT function to try would be $f(x)-f(x-1)$.
$endgroup$
– Arthur
Dec 18 '18 at 21:22
3
$begingroup$
Define $g(x)=f(x)-f(x-1)$, Then consider $g$ at $x=1$ and $x=2$ $g(1)=f(1)$ $g(2)=-f(1)$ so they have opposite sign... (well... if they don't have opposite sign then it's even easier... )
$endgroup$
– Mason
Dec 18 '18 at 21:24
$begingroup$
.Got it. Thank you!
$endgroup$
– Charith Jeewantha
Dec 18 '18 at 21:33