Show that if $f(-1)0$, then there exist $-1<a<1$ such that $f(a)=0$
$begingroup$
Let $f:mathbb{R}tomathbb{R}$ continuous. Show that if $f(-1)<0$ and $f(1)>0$, then there exist $-1<a<1$ such that $f(a)=0$.
My approach: Suppose that $f(x)neq 0$ for all $xin (-1,1)$. Then $$f((-1,1))=(f(-1),0)cup(0,f(1))qquadqquad(*)$$
Furthermore, $f$ is continuous and the image of connected set is connected, then (*) is a contradiction.
Therefore, must be exist some $ain(-1,1)$ such that $f(a)=0$. This is correct? Can give me some hint. Thanks!
real-analysis continuity connectedness
$endgroup$
add a comment |
$begingroup$
Let $f:mathbb{R}tomathbb{R}$ continuous. Show that if $f(-1)<0$ and $f(1)>0$, then there exist $-1<a<1$ such that $f(a)=0$.
My approach: Suppose that $f(x)neq 0$ for all $xin (-1,1)$. Then $$f((-1,1))=(f(-1),0)cup(0,f(1))qquadqquad(*)$$
Furthermore, $f$ is continuous and the image of connected set is connected, then (*) is a contradiction.
Therefore, must be exist some $ain(-1,1)$ such that $f(a)=0$. This is correct? Can give me some hint. Thanks!
real-analysis continuity connectedness
$endgroup$
1
$begingroup$
Would it not just be sufficient to use the intermediate value theorem?
$endgroup$
– Eevee Trainer
Dec 23 '18 at 23:16
$begingroup$
I know these theorem, but I want to demonstrate this problem without using the IVT.
$endgroup$
– user570343
Dec 23 '18 at 23:18
$begingroup$
Using the intermediate value theorem is a bit like assuming what you are to prove: the statement of the question basically is the intermediate value theorem.
$endgroup$
– Winther
Dec 23 '18 at 23:41
add a comment |
$begingroup$
Let $f:mathbb{R}tomathbb{R}$ continuous. Show that if $f(-1)<0$ and $f(1)>0$, then there exist $-1<a<1$ such that $f(a)=0$.
My approach: Suppose that $f(x)neq 0$ for all $xin (-1,1)$. Then $$f((-1,1))=(f(-1),0)cup(0,f(1))qquadqquad(*)$$
Furthermore, $f$ is continuous and the image of connected set is connected, then (*) is a contradiction.
Therefore, must be exist some $ain(-1,1)$ such that $f(a)=0$. This is correct? Can give me some hint. Thanks!
real-analysis continuity connectedness
$endgroup$
Let $f:mathbb{R}tomathbb{R}$ continuous. Show that if $f(-1)<0$ and $f(1)>0$, then there exist $-1<a<1$ such that $f(a)=0$.
My approach: Suppose that $f(x)neq 0$ for all $xin (-1,1)$. Then $$f((-1,1))=(f(-1),0)cup(0,f(1))qquadqquad(*)$$
Furthermore, $f$ is continuous and the image of connected set is connected, then (*) is a contradiction.
Therefore, must be exist some $ain(-1,1)$ such that $f(a)=0$. This is correct? Can give me some hint. Thanks!
real-analysis continuity connectedness
real-analysis continuity connectedness
edited Dec 23 '18 at 23:27
asked Dec 23 '18 at 23:11
user570343
1
$begingroup$
Would it not just be sufficient to use the intermediate value theorem?
$endgroup$
– Eevee Trainer
Dec 23 '18 at 23:16
$begingroup$
I know these theorem, but I want to demonstrate this problem without using the IVT.
$endgroup$
– user570343
Dec 23 '18 at 23:18
$begingroup$
Using the intermediate value theorem is a bit like assuming what you are to prove: the statement of the question basically is the intermediate value theorem.
$endgroup$
– Winther
Dec 23 '18 at 23:41
add a comment |
1
$begingroup$
Would it not just be sufficient to use the intermediate value theorem?
$endgroup$
– Eevee Trainer
Dec 23 '18 at 23:16
$begingroup$
I know these theorem, but I want to demonstrate this problem without using the IVT.
$endgroup$
– user570343
Dec 23 '18 at 23:18
$begingroup$
Using the intermediate value theorem is a bit like assuming what you are to prove: the statement of the question basically is the intermediate value theorem.
$endgroup$
– Winther
Dec 23 '18 at 23:41
1
1
$begingroup$
Would it not just be sufficient to use the intermediate value theorem?
$endgroup$
– Eevee Trainer
Dec 23 '18 at 23:16
$begingroup$
Would it not just be sufficient to use the intermediate value theorem?
$endgroup$
– Eevee Trainer
Dec 23 '18 at 23:16
$begingroup$
I know these theorem, but I want to demonstrate this problem without using the IVT.
$endgroup$
– user570343
Dec 23 '18 at 23:18
$begingroup$
I know these theorem, but I want to demonstrate this problem without using the IVT.
$endgroup$
– user570343
Dec 23 '18 at 23:18
$begingroup$
Using the intermediate value theorem is a bit like assuming what you are to prove: the statement of the question basically is the intermediate value theorem.
$endgroup$
– Winther
Dec 23 '18 at 23:41
$begingroup$
Using the intermediate value theorem is a bit like assuming what you are to prove: the statement of the question basically is the intermediate value theorem.
$endgroup$
– Winther
Dec 23 '18 at 23:41
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
Hint: the only connected subsets of $Bbb{R}$ are intervals (closed, open, half-closed/half-open and possibly with infinite endpoints). The image $f[[-1, 1]]$ of the closed interval $[-1, 1]$ under your function $f$ is connected and includes $f(-1) < 0$ and $f(1) > 0$, so it is an interval containing a negative number and a positive number. Can you take it from there?
$endgroup$
$begingroup$
Ohh great! Thanks, But, why my "solution" is not correct?
$endgroup$
– user570343
Dec 23 '18 at 23:28
$begingroup$
Because you are making unjustified assumptions about the behaviour of $f$. E.g. you could have $f(-1) > f(-1/2)$.
$endgroup$
– Rob Arthan
Dec 23 '18 at 23:31
$begingroup$
I don't see what implies that $f(-1)>f(-1/2)$?? I
$endgroup$
– user570343
Dec 23 '18 at 23:35
$begingroup$
If you had $f(-1) > f(-1/2)$ then your equation (*) might be false. All you need to say is that the image $f[[-1, 1]]$ is connected and includes the point $0 in [f(-1), f(1)]$. Your equation for $f[(-1, 1)]$ need not hold. (I am writing $f[X]$ for the image of the set $X$ under the function $f$ where you are writing $f((X))$.)
$endgroup$
– Rob Arthan
Dec 23 '18 at 23:44
add a comment |
$begingroup$
Just because $f(x) neq 0$ on $x in (-1,1)$ does not mean that $0$ is the only omitted value, which you assert via "$(f(-1),0) cup (0,f(1))$"${} = (f(-1),f(1)) smallsetminus {0}$. My age (in years) is continuous and has never been $-1$, but this does not mean that $-1$ is the only number my age can never be.
$endgroup$
add a comment |
$begingroup$
(*) is not correct. You are assuming that all values of $f$ are between $f(-1)$ and $f(1)$ which may not be true. Use: $f(-1,1)=[f(-1,1)cap (-infty,0)]cup [f(-1,1)cap (0,infty)]$.
$endgroup$
add a comment |
Your Answer
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Hint: the only connected subsets of $Bbb{R}$ are intervals (closed, open, half-closed/half-open and possibly with infinite endpoints). The image $f[[-1, 1]]$ of the closed interval $[-1, 1]$ under your function $f$ is connected and includes $f(-1) < 0$ and $f(1) > 0$, so it is an interval containing a negative number and a positive number. Can you take it from there?
$endgroup$
$begingroup$
Ohh great! Thanks, But, why my "solution" is not correct?
$endgroup$
– user570343
Dec 23 '18 at 23:28
$begingroup$
Because you are making unjustified assumptions about the behaviour of $f$. E.g. you could have $f(-1) > f(-1/2)$.
$endgroup$
– Rob Arthan
Dec 23 '18 at 23:31
$begingroup$
I don't see what implies that $f(-1)>f(-1/2)$?? I
$endgroup$
– user570343
Dec 23 '18 at 23:35
$begingroup$
If you had $f(-1) > f(-1/2)$ then your equation (*) might be false. All you need to say is that the image $f[[-1, 1]]$ is connected and includes the point $0 in [f(-1), f(1)]$. Your equation for $f[(-1, 1)]$ need not hold. (I am writing $f[X]$ for the image of the set $X$ under the function $f$ where you are writing $f((X))$.)
$endgroup$
– Rob Arthan
Dec 23 '18 at 23:44
add a comment |
$begingroup$
Hint: the only connected subsets of $Bbb{R}$ are intervals (closed, open, half-closed/half-open and possibly with infinite endpoints). The image $f[[-1, 1]]$ of the closed interval $[-1, 1]$ under your function $f$ is connected and includes $f(-1) < 0$ and $f(1) > 0$, so it is an interval containing a negative number and a positive number. Can you take it from there?
$endgroup$
$begingroup$
Ohh great! Thanks, But, why my "solution" is not correct?
$endgroup$
– user570343
Dec 23 '18 at 23:28
$begingroup$
Because you are making unjustified assumptions about the behaviour of $f$. E.g. you could have $f(-1) > f(-1/2)$.
$endgroup$
– Rob Arthan
Dec 23 '18 at 23:31
$begingroup$
I don't see what implies that $f(-1)>f(-1/2)$?? I
$endgroup$
– user570343
Dec 23 '18 at 23:35
$begingroup$
If you had $f(-1) > f(-1/2)$ then your equation (*) might be false. All you need to say is that the image $f[[-1, 1]]$ is connected and includes the point $0 in [f(-1), f(1)]$. Your equation for $f[(-1, 1)]$ need not hold. (I am writing $f[X]$ for the image of the set $X$ under the function $f$ where you are writing $f((X))$.)
$endgroup$
– Rob Arthan
Dec 23 '18 at 23:44
add a comment |
$begingroup$
Hint: the only connected subsets of $Bbb{R}$ are intervals (closed, open, half-closed/half-open and possibly with infinite endpoints). The image $f[[-1, 1]]$ of the closed interval $[-1, 1]$ under your function $f$ is connected and includes $f(-1) < 0$ and $f(1) > 0$, so it is an interval containing a negative number and a positive number. Can you take it from there?
$endgroup$
Hint: the only connected subsets of $Bbb{R}$ are intervals (closed, open, half-closed/half-open and possibly with infinite endpoints). The image $f[[-1, 1]]$ of the closed interval $[-1, 1]$ under your function $f$ is connected and includes $f(-1) < 0$ and $f(1) > 0$, so it is an interval containing a negative number and a positive number. Can you take it from there?
edited Dec 23 '18 at 23:35
answered Dec 23 '18 at 23:27
Rob ArthanRob Arthan
29.3k42966
29.3k42966
$begingroup$
Ohh great! Thanks, But, why my "solution" is not correct?
$endgroup$
– user570343
Dec 23 '18 at 23:28
$begingroup$
Because you are making unjustified assumptions about the behaviour of $f$. E.g. you could have $f(-1) > f(-1/2)$.
$endgroup$
– Rob Arthan
Dec 23 '18 at 23:31
$begingroup$
I don't see what implies that $f(-1)>f(-1/2)$?? I
$endgroup$
– user570343
Dec 23 '18 at 23:35
$begingroup$
If you had $f(-1) > f(-1/2)$ then your equation (*) might be false. All you need to say is that the image $f[[-1, 1]]$ is connected and includes the point $0 in [f(-1), f(1)]$. Your equation for $f[(-1, 1)]$ need not hold. (I am writing $f[X]$ for the image of the set $X$ under the function $f$ where you are writing $f((X))$.)
$endgroup$
– Rob Arthan
Dec 23 '18 at 23:44
add a comment |
$begingroup$
Ohh great! Thanks, But, why my "solution" is not correct?
$endgroup$
– user570343
Dec 23 '18 at 23:28
$begingroup$
Because you are making unjustified assumptions about the behaviour of $f$. E.g. you could have $f(-1) > f(-1/2)$.
$endgroup$
– Rob Arthan
Dec 23 '18 at 23:31
$begingroup$
I don't see what implies that $f(-1)>f(-1/2)$?? I
$endgroup$
– user570343
Dec 23 '18 at 23:35
$begingroup$
If you had $f(-1) > f(-1/2)$ then your equation (*) might be false. All you need to say is that the image $f[[-1, 1]]$ is connected and includes the point $0 in [f(-1), f(1)]$. Your equation for $f[(-1, 1)]$ need not hold. (I am writing $f[X]$ for the image of the set $X$ under the function $f$ where you are writing $f((X))$.)
$endgroup$
– Rob Arthan
Dec 23 '18 at 23:44
$begingroup$
Ohh great! Thanks, But, why my "solution" is not correct?
$endgroup$
– user570343
Dec 23 '18 at 23:28
$begingroup$
Ohh great! Thanks, But, why my "solution" is not correct?
$endgroup$
– user570343
Dec 23 '18 at 23:28
$begingroup$
Because you are making unjustified assumptions about the behaviour of $f$. E.g. you could have $f(-1) > f(-1/2)$.
$endgroup$
– Rob Arthan
Dec 23 '18 at 23:31
$begingroup$
Because you are making unjustified assumptions about the behaviour of $f$. E.g. you could have $f(-1) > f(-1/2)$.
$endgroup$
– Rob Arthan
Dec 23 '18 at 23:31
$begingroup$
I don't see what implies that $f(-1)>f(-1/2)$?? I
$endgroup$
– user570343
Dec 23 '18 at 23:35
$begingroup$
I don't see what implies that $f(-1)>f(-1/2)$?? I
$endgroup$
– user570343
Dec 23 '18 at 23:35
$begingroup$
If you had $f(-1) > f(-1/2)$ then your equation (*) might be false. All you need to say is that the image $f[[-1, 1]]$ is connected and includes the point $0 in [f(-1), f(1)]$. Your equation for $f[(-1, 1)]$ need not hold. (I am writing $f[X]$ for the image of the set $X$ under the function $f$ where you are writing $f((X))$.)
$endgroup$
– Rob Arthan
Dec 23 '18 at 23:44
$begingroup$
If you had $f(-1) > f(-1/2)$ then your equation (*) might be false. All you need to say is that the image $f[[-1, 1]]$ is connected and includes the point $0 in [f(-1), f(1)]$. Your equation for $f[(-1, 1)]$ need not hold. (I am writing $f[X]$ for the image of the set $X$ under the function $f$ where you are writing $f((X))$.)
$endgroup$
– Rob Arthan
Dec 23 '18 at 23:44
add a comment |
$begingroup$
Just because $f(x) neq 0$ on $x in (-1,1)$ does not mean that $0$ is the only omitted value, which you assert via "$(f(-1),0) cup (0,f(1))$"${} = (f(-1),f(1)) smallsetminus {0}$. My age (in years) is continuous and has never been $-1$, but this does not mean that $-1$ is the only number my age can never be.
$endgroup$
add a comment |
$begingroup$
Just because $f(x) neq 0$ on $x in (-1,1)$ does not mean that $0$ is the only omitted value, which you assert via "$(f(-1),0) cup (0,f(1))$"${} = (f(-1),f(1)) smallsetminus {0}$. My age (in years) is continuous and has never been $-1$, but this does not mean that $-1$ is the only number my age can never be.
$endgroup$
add a comment |
$begingroup$
Just because $f(x) neq 0$ on $x in (-1,1)$ does not mean that $0$ is the only omitted value, which you assert via "$(f(-1),0) cup (0,f(1))$"${} = (f(-1),f(1)) smallsetminus {0}$. My age (in years) is continuous and has never been $-1$, but this does not mean that $-1$ is the only number my age can never be.
$endgroup$
Just because $f(x) neq 0$ on $x in (-1,1)$ does not mean that $0$ is the only omitted value, which you assert via "$(f(-1),0) cup (0,f(1))$"${} = (f(-1),f(1)) smallsetminus {0}$. My age (in years) is continuous and has never been $-1$, but this does not mean that $-1$ is the only number my age can never be.
answered Dec 23 '18 at 23:19
Eric TowersEric Towers
32.7k22370
32.7k22370
add a comment |
add a comment |
$begingroup$
(*) is not correct. You are assuming that all values of $f$ are between $f(-1)$ and $f(1)$ which may not be true. Use: $f(-1,1)=[f(-1,1)cap (-infty,0)]cup [f(-1,1)cap (0,infty)]$.
$endgroup$
add a comment |
$begingroup$
(*) is not correct. You are assuming that all values of $f$ are between $f(-1)$ and $f(1)$ which may not be true. Use: $f(-1,1)=[f(-1,1)cap (-infty,0)]cup [f(-1,1)cap (0,infty)]$.
$endgroup$
add a comment |
$begingroup$
(*) is not correct. You are assuming that all values of $f$ are between $f(-1)$ and $f(1)$ which may not be true. Use: $f(-1,1)=[f(-1,1)cap (-infty,0)]cup [f(-1,1)cap (0,infty)]$.
$endgroup$
(*) is not correct. You are assuming that all values of $f$ are between $f(-1)$ and $f(1)$ which may not be true. Use: $f(-1,1)=[f(-1,1)cap (-infty,0)]cup [f(-1,1)cap (0,infty)]$.
answered Dec 23 '18 at 23:30
Kavi Rama MurthyKavi Rama Murthy
61.6k42262
61.6k42262
add a comment |
add a comment |
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1
$begingroup$
Would it not just be sufficient to use the intermediate value theorem?
$endgroup$
– Eevee Trainer
Dec 23 '18 at 23:16
$begingroup$
I know these theorem, but I want to demonstrate this problem without using the IVT.
$endgroup$
– user570343
Dec 23 '18 at 23:18
$begingroup$
Using the intermediate value theorem is a bit like assuming what you are to prove: the statement of the question basically is the intermediate value theorem.
$endgroup$
– Winther
Dec 23 '18 at 23:41