Prove that if $f(x)=f(y)$ for all $fin X^{*},$ then $x=y$
$begingroup$
Can you check if my proof is correct?
Let $X$ be a normed linear space. Prove that if $f(x)=f(y)$ for all $fin X^{*},$ then $x=y$
Let $fin X^{*}$, then $f$ is a bounded linear functional. Assume that $x,in X$ such that begin{align}f(x)=f(y)&iff f(x)-f(y)=0, \& iff f(x-y)=0, ;text{since};f ;text{is a linear functional};\& iff x-yin ker f ={0}\& iff x=yend{align}
linear-algebra functional-analysis analysis
$endgroup$
|
show 1 more comment
$begingroup$
Can you check if my proof is correct?
Let $X$ be a normed linear space. Prove that if $f(x)=f(y)$ for all $fin X^{*},$ then $x=y$
Let $fin X^{*}$, then $f$ is a bounded linear functional. Assume that $x,in X$ such that begin{align}f(x)=f(y)&iff f(x)-f(y)=0, \& iff f(x-y)=0, ;text{since};f ;text{is a linear functional};\& iff x-yin ker f ={0}\& iff x=yend{align}
linear-algebra functional-analysis analysis
$endgroup$
1
$begingroup$
Does $X^*$ stand for the set of bounded linear functionals, or for the set of all linear functionals? And do you really believe $ker f = {0}$ when $fcolon XtoBbb C$ (or $Bbb R$)?
$endgroup$
– Ted Shifrin
Jan 10 at 19:25
$begingroup$
@Ted Shifrin: $X^{*}$ stands for the set of bounded linear functionals
$endgroup$
– Omojola Micheal
Jan 10 at 19:27
$begingroup$
@Ted Shifrin: However, I am not certain if $ker f={0}$ when $f:Xto Bbb{R}.$ What do you think?
$endgroup$
– Omojola Micheal
Jan 10 at 19:29
$begingroup$
LOL, What if $X=Bbb R^n$? What's the kernel then?
$endgroup$
– Ted Shifrin
Jan 10 at 19:29
1
$begingroup$
Do you know the nullity-rank theorem? ... The key thing you're not using here is that if holds for all $fin X^*$. Try this in $Bbb R^n$: Suppose $xcdot v = 0$ for all $vinBbb R^n$. Why must $x=0$?
$endgroup$
– Ted Shifrin
Jan 10 at 19:31
|
show 1 more comment
$begingroup$
Can you check if my proof is correct?
Let $X$ be a normed linear space. Prove that if $f(x)=f(y)$ for all $fin X^{*},$ then $x=y$
Let $fin X^{*}$, then $f$ is a bounded linear functional. Assume that $x,in X$ such that begin{align}f(x)=f(y)&iff f(x)-f(y)=0, \& iff f(x-y)=0, ;text{since};f ;text{is a linear functional};\& iff x-yin ker f ={0}\& iff x=yend{align}
linear-algebra functional-analysis analysis
$endgroup$
Can you check if my proof is correct?
Let $X$ be a normed linear space. Prove that if $f(x)=f(y)$ for all $fin X^{*},$ then $x=y$
Let $fin X^{*}$, then $f$ is a bounded linear functional. Assume that $x,in X$ such that begin{align}f(x)=f(y)&iff f(x)-f(y)=0, \& iff f(x-y)=0, ;text{since};f ;text{is a linear functional};\& iff x-yin ker f ={0}\& iff x=yend{align}
linear-algebra functional-analysis analysis
linear-algebra functional-analysis analysis
edited Jan 10 at 19:26
Omojola Micheal
asked Jan 10 at 19:23
Omojola MichealOmojola Micheal
1,831324
1,831324
1
$begingroup$
Does $X^*$ stand for the set of bounded linear functionals, or for the set of all linear functionals? And do you really believe $ker f = {0}$ when $fcolon XtoBbb C$ (or $Bbb R$)?
$endgroup$
– Ted Shifrin
Jan 10 at 19:25
$begingroup$
@Ted Shifrin: $X^{*}$ stands for the set of bounded linear functionals
$endgroup$
– Omojola Micheal
Jan 10 at 19:27
$begingroup$
@Ted Shifrin: However, I am not certain if $ker f={0}$ when $f:Xto Bbb{R}.$ What do you think?
$endgroup$
– Omojola Micheal
Jan 10 at 19:29
$begingroup$
LOL, What if $X=Bbb R^n$? What's the kernel then?
$endgroup$
– Ted Shifrin
Jan 10 at 19:29
1
$begingroup$
Do you know the nullity-rank theorem? ... The key thing you're not using here is that if holds for all $fin X^*$. Try this in $Bbb R^n$: Suppose $xcdot v = 0$ for all $vinBbb R^n$. Why must $x=0$?
$endgroup$
– Ted Shifrin
Jan 10 at 19:31
|
show 1 more comment
1
$begingroup$
Does $X^*$ stand for the set of bounded linear functionals, or for the set of all linear functionals? And do you really believe $ker f = {0}$ when $fcolon XtoBbb C$ (or $Bbb R$)?
$endgroup$
– Ted Shifrin
Jan 10 at 19:25
$begingroup$
@Ted Shifrin: $X^{*}$ stands for the set of bounded linear functionals
$endgroup$
– Omojola Micheal
Jan 10 at 19:27
$begingroup$
@Ted Shifrin: However, I am not certain if $ker f={0}$ when $f:Xto Bbb{R}.$ What do you think?
$endgroup$
– Omojola Micheal
Jan 10 at 19:29
$begingroup$
LOL, What if $X=Bbb R^n$? What's the kernel then?
$endgroup$
– Ted Shifrin
Jan 10 at 19:29
1
$begingroup$
Do you know the nullity-rank theorem? ... The key thing you're not using here is that if holds for all $fin X^*$. Try this in $Bbb R^n$: Suppose $xcdot v = 0$ for all $vinBbb R^n$. Why must $x=0$?
$endgroup$
– Ted Shifrin
Jan 10 at 19:31
1
1
$begingroup$
Does $X^*$ stand for the set of bounded linear functionals, or for the set of all linear functionals? And do you really believe $ker f = {0}$ when $fcolon XtoBbb C$ (or $Bbb R$)?
$endgroup$
– Ted Shifrin
Jan 10 at 19:25
$begingroup$
Does $X^*$ stand for the set of bounded linear functionals, or for the set of all linear functionals? And do you really believe $ker f = {0}$ when $fcolon XtoBbb C$ (or $Bbb R$)?
$endgroup$
– Ted Shifrin
Jan 10 at 19:25
$begingroup$
@Ted Shifrin: $X^{*}$ stands for the set of bounded linear functionals
$endgroup$
– Omojola Micheal
Jan 10 at 19:27
$begingroup$
@Ted Shifrin: $X^{*}$ stands for the set of bounded linear functionals
$endgroup$
– Omojola Micheal
Jan 10 at 19:27
$begingroup$
@Ted Shifrin: However, I am not certain if $ker f={0}$ when $f:Xto Bbb{R}.$ What do you think?
$endgroup$
– Omojola Micheal
Jan 10 at 19:29
$begingroup$
@Ted Shifrin: However, I am not certain if $ker f={0}$ when $f:Xto Bbb{R}.$ What do you think?
$endgroup$
– Omojola Micheal
Jan 10 at 19:29
$begingroup$
LOL, What if $X=Bbb R^n$? What's the kernel then?
$endgroup$
– Ted Shifrin
Jan 10 at 19:29
$begingroup$
LOL, What if $X=Bbb R^n$? What's the kernel then?
$endgroup$
– Ted Shifrin
Jan 10 at 19:29
1
1
$begingroup$
Do you know the nullity-rank theorem? ... The key thing you're not using here is that if holds for all $fin X^*$. Try this in $Bbb R^n$: Suppose $xcdot v = 0$ for all $vinBbb R^n$. Why must $x=0$?
$endgroup$
– Ted Shifrin
Jan 10 at 19:31
$begingroup$
Do you know the nullity-rank theorem? ... The key thing you're not using here is that if holds for all $fin X^*$. Try this in $Bbb R^n$: Suppose $xcdot v = 0$ for all $vinBbb R^n$. Why must $x=0$?
$endgroup$
– Ted Shifrin
Jan 10 at 19:31
|
show 1 more comment
2 Answers
2
active
oldest
votes
$begingroup$
The statement is true, but the proof is not. In the proof, it is implicitly assumed that if $fin X^*$, then $ker f={0}$, which is not true in general (think of the zero functional).
A correct proof can be constructed using the Hahn–Banach theorem. In particular, if $x-yneq 0$, then there exists some $fin X^*$ such that $f(x-y)=|x-y|neq 0$, so that $f(x)neq f(y)$. For details, see Theorem 5.8(b) in Folland (1999, p. 159).
$endgroup$
$begingroup$
Thanks for your quick response. I'll read through (+1)
$endgroup$
– Omojola Micheal
Jan 10 at 19:38
$begingroup$
I can't access the book. Is there anyway of getting it? Or can you share with me on my email?
$endgroup$
– Omojola Micheal
Jan 10 at 19:48
$begingroup$
The relevant portions may be accessible as a preview through Google Books or Amazon. There is also a wide variety of electronic resources available free of charge. See, for example, Proposition 6.5 in this handout.
$endgroup$
– triple_sec
Jan 10 at 20:05
$begingroup$
Yes, that's true!
$endgroup$
– Omojola Micheal
Jan 10 at 20:10
add a comment |
$begingroup$
Your proof is not correct, because, unles $dim Xleqslant1$, $ker f$ cannot possibly be ${0}$.
Let $z=x-y$ and let $Z$ be the vector space spanned by $z$. Consider the linear map$$begin{array}{rccc}gcolon&Z&longrightarrow&mathbb R\&lambda z&mapsto&lambda.end{array}$$Then $g$ is bounded and therefore, by the Hahn-Banach theorem, you can extend it to an element $fin X^*$. Butbegin{align}f(x)-f(y)&=f(x-y)\&=f(z)\&=g(z)\&=1\&neq0.end{align}
$endgroup$
$begingroup$
Thanks for your quick response. (+1)
$endgroup$
– Omojola Micheal
Jan 10 at 19:37
add a comment |
Your Answer
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2 Answers
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2 Answers
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$begingroup$
The statement is true, but the proof is not. In the proof, it is implicitly assumed that if $fin X^*$, then $ker f={0}$, which is not true in general (think of the zero functional).
A correct proof can be constructed using the Hahn–Banach theorem. In particular, if $x-yneq 0$, then there exists some $fin X^*$ such that $f(x-y)=|x-y|neq 0$, so that $f(x)neq f(y)$. For details, see Theorem 5.8(b) in Folland (1999, p. 159).
$endgroup$
$begingroup$
Thanks for your quick response. I'll read through (+1)
$endgroup$
– Omojola Micheal
Jan 10 at 19:38
$begingroup$
I can't access the book. Is there anyway of getting it? Or can you share with me on my email?
$endgroup$
– Omojola Micheal
Jan 10 at 19:48
$begingroup$
The relevant portions may be accessible as a preview through Google Books or Amazon. There is also a wide variety of electronic resources available free of charge. See, for example, Proposition 6.5 in this handout.
$endgroup$
– triple_sec
Jan 10 at 20:05
$begingroup$
Yes, that's true!
$endgroup$
– Omojola Micheal
Jan 10 at 20:10
add a comment |
$begingroup$
The statement is true, but the proof is not. In the proof, it is implicitly assumed that if $fin X^*$, then $ker f={0}$, which is not true in general (think of the zero functional).
A correct proof can be constructed using the Hahn–Banach theorem. In particular, if $x-yneq 0$, then there exists some $fin X^*$ such that $f(x-y)=|x-y|neq 0$, so that $f(x)neq f(y)$. For details, see Theorem 5.8(b) in Folland (1999, p. 159).
$endgroup$
$begingroup$
Thanks for your quick response. I'll read through (+1)
$endgroup$
– Omojola Micheal
Jan 10 at 19:38
$begingroup$
I can't access the book. Is there anyway of getting it? Or can you share with me on my email?
$endgroup$
– Omojola Micheal
Jan 10 at 19:48
$begingroup$
The relevant portions may be accessible as a preview through Google Books or Amazon. There is also a wide variety of electronic resources available free of charge. See, for example, Proposition 6.5 in this handout.
$endgroup$
– triple_sec
Jan 10 at 20:05
$begingroup$
Yes, that's true!
$endgroup$
– Omojola Micheal
Jan 10 at 20:10
add a comment |
$begingroup$
The statement is true, but the proof is not. In the proof, it is implicitly assumed that if $fin X^*$, then $ker f={0}$, which is not true in general (think of the zero functional).
A correct proof can be constructed using the Hahn–Banach theorem. In particular, if $x-yneq 0$, then there exists some $fin X^*$ such that $f(x-y)=|x-y|neq 0$, so that $f(x)neq f(y)$. For details, see Theorem 5.8(b) in Folland (1999, p. 159).
$endgroup$
The statement is true, but the proof is not. In the proof, it is implicitly assumed that if $fin X^*$, then $ker f={0}$, which is not true in general (think of the zero functional).
A correct proof can be constructed using the Hahn–Banach theorem. In particular, if $x-yneq 0$, then there exists some $fin X^*$ such that $f(x-y)=|x-y|neq 0$, so that $f(x)neq f(y)$. For details, see Theorem 5.8(b) in Folland (1999, p. 159).
answered Jan 10 at 19:36
triple_sectriple_sec
15.8k21851
15.8k21851
$begingroup$
Thanks for your quick response. I'll read through (+1)
$endgroup$
– Omojola Micheal
Jan 10 at 19:38
$begingroup$
I can't access the book. Is there anyway of getting it? Or can you share with me on my email?
$endgroup$
– Omojola Micheal
Jan 10 at 19:48
$begingroup$
The relevant portions may be accessible as a preview through Google Books or Amazon. There is also a wide variety of electronic resources available free of charge. See, for example, Proposition 6.5 in this handout.
$endgroup$
– triple_sec
Jan 10 at 20:05
$begingroup$
Yes, that's true!
$endgroup$
– Omojola Micheal
Jan 10 at 20:10
add a comment |
$begingroup$
Thanks for your quick response. I'll read through (+1)
$endgroup$
– Omojola Micheal
Jan 10 at 19:38
$begingroup$
I can't access the book. Is there anyway of getting it? Or can you share with me on my email?
$endgroup$
– Omojola Micheal
Jan 10 at 19:48
$begingroup$
The relevant portions may be accessible as a preview through Google Books or Amazon. There is also a wide variety of electronic resources available free of charge. See, for example, Proposition 6.5 in this handout.
$endgroup$
– triple_sec
Jan 10 at 20:05
$begingroup$
Yes, that's true!
$endgroup$
– Omojola Micheal
Jan 10 at 20:10
$begingroup$
Thanks for your quick response. I'll read through (+1)
$endgroup$
– Omojola Micheal
Jan 10 at 19:38
$begingroup$
Thanks for your quick response. I'll read through (+1)
$endgroup$
– Omojola Micheal
Jan 10 at 19:38
$begingroup$
I can't access the book. Is there anyway of getting it? Or can you share with me on my email?
$endgroup$
– Omojola Micheal
Jan 10 at 19:48
$begingroup$
I can't access the book. Is there anyway of getting it? Or can you share with me on my email?
$endgroup$
– Omojola Micheal
Jan 10 at 19:48
$begingroup$
The relevant portions may be accessible as a preview through Google Books or Amazon. There is also a wide variety of electronic resources available free of charge. See, for example, Proposition 6.5 in this handout.
$endgroup$
– triple_sec
Jan 10 at 20:05
$begingroup$
The relevant portions may be accessible as a preview through Google Books or Amazon. There is also a wide variety of electronic resources available free of charge. See, for example, Proposition 6.5 in this handout.
$endgroup$
– triple_sec
Jan 10 at 20:05
$begingroup$
Yes, that's true!
$endgroup$
– Omojola Micheal
Jan 10 at 20:10
$begingroup$
Yes, that's true!
$endgroup$
– Omojola Micheal
Jan 10 at 20:10
add a comment |
$begingroup$
Your proof is not correct, because, unles $dim Xleqslant1$, $ker f$ cannot possibly be ${0}$.
Let $z=x-y$ and let $Z$ be the vector space spanned by $z$. Consider the linear map$$begin{array}{rccc}gcolon&Z&longrightarrow&mathbb R\&lambda z&mapsto&lambda.end{array}$$Then $g$ is bounded and therefore, by the Hahn-Banach theorem, you can extend it to an element $fin X^*$. Butbegin{align}f(x)-f(y)&=f(x-y)\&=f(z)\&=g(z)\&=1\&neq0.end{align}
$endgroup$
$begingroup$
Thanks for your quick response. (+1)
$endgroup$
– Omojola Micheal
Jan 10 at 19:37
add a comment |
$begingroup$
Your proof is not correct, because, unles $dim Xleqslant1$, $ker f$ cannot possibly be ${0}$.
Let $z=x-y$ and let $Z$ be the vector space spanned by $z$. Consider the linear map$$begin{array}{rccc}gcolon&Z&longrightarrow&mathbb R\&lambda z&mapsto&lambda.end{array}$$Then $g$ is bounded and therefore, by the Hahn-Banach theorem, you can extend it to an element $fin X^*$. Butbegin{align}f(x)-f(y)&=f(x-y)\&=f(z)\&=g(z)\&=1\&neq0.end{align}
$endgroup$
$begingroup$
Thanks for your quick response. (+1)
$endgroup$
– Omojola Micheal
Jan 10 at 19:37
add a comment |
$begingroup$
Your proof is not correct, because, unles $dim Xleqslant1$, $ker f$ cannot possibly be ${0}$.
Let $z=x-y$ and let $Z$ be the vector space spanned by $z$. Consider the linear map$$begin{array}{rccc}gcolon&Z&longrightarrow&mathbb R\&lambda z&mapsto&lambda.end{array}$$Then $g$ is bounded and therefore, by the Hahn-Banach theorem, you can extend it to an element $fin X^*$. Butbegin{align}f(x)-f(y)&=f(x-y)\&=f(z)\&=g(z)\&=1\&neq0.end{align}
$endgroup$
Your proof is not correct, because, unles $dim Xleqslant1$, $ker f$ cannot possibly be ${0}$.
Let $z=x-y$ and let $Z$ be the vector space spanned by $z$. Consider the linear map$$begin{array}{rccc}gcolon&Z&longrightarrow&mathbb R\&lambda z&mapsto&lambda.end{array}$$Then $g$ is bounded and therefore, by the Hahn-Banach theorem, you can extend it to an element $fin X^*$. Butbegin{align}f(x)-f(y)&=f(x-y)\&=f(z)\&=g(z)\&=1\&neq0.end{align}
answered Jan 10 at 19:34
José Carlos SantosJosé Carlos Santos
156k22125227
156k22125227
$begingroup$
Thanks for your quick response. (+1)
$endgroup$
– Omojola Micheal
Jan 10 at 19:37
add a comment |
$begingroup$
Thanks for your quick response. (+1)
$endgroup$
– Omojola Micheal
Jan 10 at 19:37
$begingroup$
Thanks for your quick response. (+1)
$endgroup$
– Omojola Micheal
Jan 10 at 19:37
$begingroup$
Thanks for your quick response. (+1)
$endgroup$
– Omojola Micheal
Jan 10 at 19:37
add a comment |
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$begingroup$
Does $X^*$ stand for the set of bounded linear functionals, or for the set of all linear functionals? And do you really believe $ker f = {0}$ when $fcolon XtoBbb C$ (or $Bbb R$)?
$endgroup$
– Ted Shifrin
Jan 10 at 19:25
$begingroup$
@Ted Shifrin: $X^{*}$ stands for the set of bounded linear functionals
$endgroup$
– Omojola Micheal
Jan 10 at 19:27
$begingroup$
@Ted Shifrin: However, I am not certain if $ker f={0}$ when $f:Xto Bbb{R}.$ What do you think?
$endgroup$
– Omojola Micheal
Jan 10 at 19:29
$begingroup$
LOL, What if $X=Bbb R^n$? What's the kernel then?
$endgroup$
– Ted Shifrin
Jan 10 at 19:29
1
$begingroup$
Do you know the nullity-rank theorem? ... The key thing you're not using here is that if holds for all $fin X^*$. Try this in $Bbb R^n$: Suppose $xcdot v = 0$ for all $vinBbb R^n$. Why must $x=0$?
$endgroup$
– Ted Shifrin
Jan 10 at 19:31