Equality of finite signed measures by showing that the integrals of every bounded continuous function...
In order to show the uniqueness of the Fourier coefficients of a signed measure, I need to show that :
For any two finite signed measures $nu_1, nu_2$ on $left([-pi, pi], mathcal{B}_{[-pi, pi]}right)$ such that : $int fmathrm dnu_1=int fmathrm dnu_2$ holds for every $f$ continuous (and bounded), we have: $nu_1=nu_2$.
This property holds for finite positive measures (Approximation of bounded measurable functions with continuous functions) because the equality of integrals for a continuous function implies that of integrals of a measurable bounded function.
The answers in this thread are based on the fact that for $mu, mu'$ probability measures, $mathcal{C}={Binmathcal{B}_{[-pi, pi]} : mu(B)=mu'(B)}$ is a $pi$-system.
Does this even hold here ?
EDIT: It does when $nu_1([-pi, pi])=nu_2([-pi, pi])$, (required to show that ${B∈mathcal{B}_{[−π,π]}:ν_1(B)=ν_2(B)}$ is a Dynkin system), which is true in my case, since $hat{ν_1}=hat{ν_2}$.
measure-theory signed-measures
add a comment |
In order to show the uniqueness of the Fourier coefficients of a signed measure, I need to show that :
For any two finite signed measures $nu_1, nu_2$ on $left([-pi, pi], mathcal{B}_{[-pi, pi]}right)$ such that : $int fmathrm dnu_1=int fmathrm dnu_2$ holds for every $f$ continuous (and bounded), we have: $nu_1=nu_2$.
This property holds for finite positive measures (Approximation of bounded measurable functions with continuous functions) because the equality of integrals for a continuous function implies that of integrals of a measurable bounded function.
The answers in this thread are based on the fact that for $mu, mu'$ probability measures, $mathcal{C}={Binmathcal{B}_{[-pi, pi]} : mu(B)=mu'(B)}$ is a $pi$-system.
Does this even hold here ?
EDIT: It does when $nu_1([-pi, pi])=nu_2([-pi, pi])$, (required to show that ${B∈mathcal{B}_{[−π,π]}:ν_1(B)=ν_2(B)}$ is a Dynkin system), which is true in my case, since $hat{ν_1}=hat{ν_2}$.
measure-theory signed-measures
Do you know the Riesz–Markov–Kakutani representation theorem, stating that $mathcal M(X)simeq C(X)^*$?
– Federico
Dec 4 '18 at 21:45
Nope, that seems out of my reach for now. Plus the material I'm following didn't even feel the need to highlight the equality, so I thought it was going to be similar to the positive case.
– deque
Dec 4 '18 at 21:58
add a comment |
In order to show the uniqueness of the Fourier coefficients of a signed measure, I need to show that :
For any two finite signed measures $nu_1, nu_2$ on $left([-pi, pi], mathcal{B}_{[-pi, pi]}right)$ such that : $int fmathrm dnu_1=int fmathrm dnu_2$ holds for every $f$ continuous (and bounded), we have: $nu_1=nu_2$.
This property holds for finite positive measures (Approximation of bounded measurable functions with continuous functions) because the equality of integrals for a continuous function implies that of integrals of a measurable bounded function.
The answers in this thread are based on the fact that for $mu, mu'$ probability measures, $mathcal{C}={Binmathcal{B}_{[-pi, pi]} : mu(B)=mu'(B)}$ is a $pi$-system.
Does this even hold here ?
EDIT: It does when $nu_1([-pi, pi])=nu_2([-pi, pi])$, (required to show that ${B∈mathcal{B}_{[−π,π]}:ν_1(B)=ν_2(B)}$ is a Dynkin system), which is true in my case, since $hat{ν_1}=hat{ν_2}$.
measure-theory signed-measures
In order to show the uniqueness of the Fourier coefficients of a signed measure, I need to show that :
For any two finite signed measures $nu_1, nu_2$ on $left([-pi, pi], mathcal{B}_{[-pi, pi]}right)$ such that : $int fmathrm dnu_1=int fmathrm dnu_2$ holds for every $f$ continuous (and bounded), we have: $nu_1=nu_2$.
This property holds for finite positive measures (Approximation of bounded measurable functions with continuous functions) because the equality of integrals for a continuous function implies that of integrals of a measurable bounded function.
The answers in this thread are based on the fact that for $mu, mu'$ probability measures, $mathcal{C}={Binmathcal{B}_{[-pi, pi]} : mu(B)=mu'(B)}$ is a $pi$-system.
Does this even hold here ?
EDIT: It does when $nu_1([-pi, pi])=nu_2([-pi, pi])$, (required to show that ${B∈mathcal{B}_{[−π,π]}:ν_1(B)=ν_2(B)}$ is a Dynkin system), which is true in my case, since $hat{ν_1}=hat{ν_2}$.
measure-theory signed-measures
measure-theory signed-measures
edited Dec 5 '18 at 22:42
deque
asked Dec 4 '18 at 21:38
dequedeque
413111
413111
Do you know the Riesz–Markov–Kakutani representation theorem, stating that $mathcal M(X)simeq C(X)^*$?
– Federico
Dec 4 '18 at 21:45
Nope, that seems out of my reach for now. Plus the material I'm following didn't even feel the need to highlight the equality, so I thought it was going to be similar to the positive case.
– deque
Dec 4 '18 at 21:58
add a comment |
Do you know the Riesz–Markov–Kakutani representation theorem, stating that $mathcal M(X)simeq C(X)^*$?
– Federico
Dec 4 '18 at 21:45
Nope, that seems out of my reach for now. Plus the material I'm following didn't even feel the need to highlight the equality, so I thought it was going to be similar to the positive case.
– deque
Dec 4 '18 at 21:58
Do you know the Riesz–Markov–Kakutani representation theorem, stating that $mathcal M(X)simeq C(X)^*$?
– Federico
Dec 4 '18 at 21:45
Do you know the Riesz–Markov–Kakutani representation theorem, stating that $mathcal M(X)simeq C(X)^*$?
– Federico
Dec 4 '18 at 21:45
Nope, that seems out of my reach for now. Plus the material I'm following didn't even feel the need to highlight the equality, so I thought it was going to be similar to the positive case.
– deque
Dec 4 '18 at 21:58
Nope, that seems out of my reach for now. Plus the material I'm following didn't even feel the need to highlight the equality, so I thought it was going to be similar to the positive case.
– deque
Dec 4 '18 at 21:58
add a comment |
1 Answer
1
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I suppose that both signed measures are finite, since otherwise we cannot expect that the integral over arbitrary continuous functions exists. You can approximate any interval $[a,b]$ by (for example) the sequence
$$g_n(x) := begin{cases} 1 & text{ if } x in [a,b] \ nx + (1-na) &text{ if } x in [a-1/n,a] \ -nx +(1+nb) & text{ if } x in [b,b+1/n] \ 0 & text{otherwise} end{cases}.$$
By the dominated convergence theorem (for signed measures) we get
$$nu_1([a,b]) =nu_2([a,b]).$$
Thus, both measures are equal on a $cap$-stable generator of the Borel-$sigma$-algebra. Now, you need to extend the uniqueness theorem for $sigma$-finite measures. In fact, the same proof applies. Note that $$mathcal{C}={Binmathcal{B}_{[-pi, pi]} : nu_1(B)=nu_2(B)}$$ is a $pi$-system containing a $cap$-stable generator with a sequence $E_n in mathcal{C}$ such that $E_n uparrow Omega$.
For completeness, we should give reasons for the application of the dominated convergence theorem: By Jordan's decomposition theorem, you can write for a signed measure $mu = mu_1 - mu_2$ with measures $mu_1$ and $mu_2$. (Moreover, there exists a 'minimal decomposition' - called the Jordan decompisition.)
Of course the measures involved here are finite, I edited that, sorry. The only problem remaining is that I can't prove that $mathcal{C}$ is stable under intersection. I managed to do it when the measures are of probability, but since they don't have the same total mass, I couldn't do it
– deque
Dec 5 '18 at 8:43
In the uniqueness theorem for $sigma$-finite measures you need that there exists a sequence $(E_n)$ in $mathcal{C}$ with $E_n uparrow Omega$. Of course, we need this here, too! And it is satisfied as we can take the constant function $f=1$ to get $nu_1(Omega) = nu_2(Omega)$.
– p4sch
Dec 5 '18 at 8:50
By "uniqueness theorem for σ-finite measures" you mean the monotone class theorem right (although it doesn't require the existence of such a sequence)? Because that was the first approach I tried, but as I said, I couldn't prove that $mathcal{C}$ is closed under intersections.
– deque
Dec 5 '18 at 8:58
1
I mean this theorem. I called it 'uniqueness theorem for $sigma$-finite measures'.
– p4sch
Dec 5 '18 at 9:12
This theorem still requires to show that $mathcal{C}$ is closed under intersections though :/ Can you help me with that please ?
– deque
Dec 5 '18 at 11:47
|
show 3 more comments
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I suppose that both signed measures are finite, since otherwise we cannot expect that the integral over arbitrary continuous functions exists. You can approximate any interval $[a,b]$ by (for example) the sequence
$$g_n(x) := begin{cases} 1 & text{ if } x in [a,b] \ nx + (1-na) &text{ if } x in [a-1/n,a] \ -nx +(1+nb) & text{ if } x in [b,b+1/n] \ 0 & text{otherwise} end{cases}.$$
By the dominated convergence theorem (for signed measures) we get
$$nu_1([a,b]) =nu_2([a,b]).$$
Thus, both measures are equal on a $cap$-stable generator of the Borel-$sigma$-algebra. Now, you need to extend the uniqueness theorem for $sigma$-finite measures. In fact, the same proof applies. Note that $$mathcal{C}={Binmathcal{B}_{[-pi, pi]} : nu_1(B)=nu_2(B)}$$ is a $pi$-system containing a $cap$-stable generator with a sequence $E_n in mathcal{C}$ such that $E_n uparrow Omega$.
For completeness, we should give reasons for the application of the dominated convergence theorem: By Jordan's decomposition theorem, you can write for a signed measure $mu = mu_1 - mu_2$ with measures $mu_1$ and $mu_2$. (Moreover, there exists a 'minimal decomposition' - called the Jordan decompisition.)
Of course the measures involved here are finite, I edited that, sorry. The only problem remaining is that I can't prove that $mathcal{C}$ is stable under intersection. I managed to do it when the measures are of probability, but since they don't have the same total mass, I couldn't do it
– deque
Dec 5 '18 at 8:43
In the uniqueness theorem for $sigma$-finite measures you need that there exists a sequence $(E_n)$ in $mathcal{C}$ with $E_n uparrow Omega$. Of course, we need this here, too! And it is satisfied as we can take the constant function $f=1$ to get $nu_1(Omega) = nu_2(Omega)$.
– p4sch
Dec 5 '18 at 8:50
By "uniqueness theorem for σ-finite measures" you mean the monotone class theorem right (although it doesn't require the existence of such a sequence)? Because that was the first approach I tried, but as I said, I couldn't prove that $mathcal{C}$ is closed under intersections.
– deque
Dec 5 '18 at 8:58
1
I mean this theorem. I called it 'uniqueness theorem for $sigma$-finite measures'.
– p4sch
Dec 5 '18 at 9:12
This theorem still requires to show that $mathcal{C}$ is closed under intersections though :/ Can you help me with that please ?
– deque
Dec 5 '18 at 11:47
|
show 3 more comments
I suppose that both signed measures are finite, since otherwise we cannot expect that the integral over arbitrary continuous functions exists. You can approximate any interval $[a,b]$ by (for example) the sequence
$$g_n(x) := begin{cases} 1 & text{ if } x in [a,b] \ nx + (1-na) &text{ if } x in [a-1/n,a] \ -nx +(1+nb) & text{ if } x in [b,b+1/n] \ 0 & text{otherwise} end{cases}.$$
By the dominated convergence theorem (for signed measures) we get
$$nu_1([a,b]) =nu_2([a,b]).$$
Thus, both measures are equal on a $cap$-stable generator of the Borel-$sigma$-algebra. Now, you need to extend the uniqueness theorem for $sigma$-finite measures. In fact, the same proof applies. Note that $$mathcal{C}={Binmathcal{B}_{[-pi, pi]} : nu_1(B)=nu_2(B)}$$ is a $pi$-system containing a $cap$-stable generator with a sequence $E_n in mathcal{C}$ such that $E_n uparrow Omega$.
For completeness, we should give reasons for the application of the dominated convergence theorem: By Jordan's decomposition theorem, you can write for a signed measure $mu = mu_1 - mu_2$ with measures $mu_1$ and $mu_2$. (Moreover, there exists a 'minimal decomposition' - called the Jordan decompisition.)
Of course the measures involved here are finite, I edited that, sorry. The only problem remaining is that I can't prove that $mathcal{C}$ is stable under intersection. I managed to do it when the measures are of probability, but since they don't have the same total mass, I couldn't do it
– deque
Dec 5 '18 at 8:43
In the uniqueness theorem for $sigma$-finite measures you need that there exists a sequence $(E_n)$ in $mathcal{C}$ with $E_n uparrow Omega$. Of course, we need this here, too! And it is satisfied as we can take the constant function $f=1$ to get $nu_1(Omega) = nu_2(Omega)$.
– p4sch
Dec 5 '18 at 8:50
By "uniqueness theorem for σ-finite measures" you mean the monotone class theorem right (although it doesn't require the existence of such a sequence)? Because that was the first approach I tried, but as I said, I couldn't prove that $mathcal{C}$ is closed under intersections.
– deque
Dec 5 '18 at 8:58
1
I mean this theorem. I called it 'uniqueness theorem for $sigma$-finite measures'.
– p4sch
Dec 5 '18 at 9:12
This theorem still requires to show that $mathcal{C}$ is closed under intersections though :/ Can you help me with that please ?
– deque
Dec 5 '18 at 11:47
|
show 3 more comments
I suppose that both signed measures are finite, since otherwise we cannot expect that the integral over arbitrary continuous functions exists. You can approximate any interval $[a,b]$ by (for example) the sequence
$$g_n(x) := begin{cases} 1 & text{ if } x in [a,b] \ nx + (1-na) &text{ if } x in [a-1/n,a] \ -nx +(1+nb) & text{ if } x in [b,b+1/n] \ 0 & text{otherwise} end{cases}.$$
By the dominated convergence theorem (for signed measures) we get
$$nu_1([a,b]) =nu_2([a,b]).$$
Thus, both measures are equal on a $cap$-stable generator of the Borel-$sigma$-algebra. Now, you need to extend the uniqueness theorem for $sigma$-finite measures. In fact, the same proof applies. Note that $$mathcal{C}={Binmathcal{B}_{[-pi, pi]} : nu_1(B)=nu_2(B)}$$ is a $pi$-system containing a $cap$-stable generator with a sequence $E_n in mathcal{C}$ such that $E_n uparrow Omega$.
For completeness, we should give reasons for the application of the dominated convergence theorem: By Jordan's decomposition theorem, you can write for a signed measure $mu = mu_1 - mu_2$ with measures $mu_1$ and $mu_2$. (Moreover, there exists a 'minimal decomposition' - called the Jordan decompisition.)
I suppose that both signed measures are finite, since otherwise we cannot expect that the integral over arbitrary continuous functions exists. You can approximate any interval $[a,b]$ by (for example) the sequence
$$g_n(x) := begin{cases} 1 & text{ if } x in [a,b] \ nx + (1-na) &text{ if } x in [a-1/n,a] \ -nx +(1+nb) & text{ if } x in [b,b+1/n] \ 0 & text{otherwise} end{cases}.$$
By the dominated convergence theorem (for signed measures) we get
$$nu_1([a,b]) =nu_2([a,b]).$$
Thus, both measures are equal on a $cap$-stable generator of the Borel-$sigma$-algebra. Now, you need to extend the uniqueness theorem for $sigma$-finite measures. In fact, the same proof applies. Note that $$mathcal{C}={Binmathcal{B}_{[-pi, pi]} : nu_1(B)=nu_2(B)}$$ is a $pi$-system containing a $cap$-stable generator with a sequence $E_n in mathcal{C}$ such that $E_n uparrow Omega$.
For completeness, we should give reasons for the application of the dominated convergence theorem: By Jordan's decomposition theorem, you can write for a signed measure $mu = mu_1 - mu_2$ with measures $mu_1$ and $mu_2$. (Moreover, there exists a 'minimal decomposition' - called the Jordan decompisition.)
edited Dec 5 '18 at 8:48
answered Dec 5 '18 at 8:04
p4schp4sch
4,770217
4,770217
Of course the measures involved here are finite, I edited that, sorry. The only problem remaining is that I can't prove that $mathcal{C}$ is stable under intersection. I managed to do it when the measures are of probability, but since they don't have the same total mass, I couldn't do it
– deque
Dec 5 '18 at 8:43
In the uniqueness theorem for $sigma$-finite measures you need that there exists a sequence $(E_n)$ in $mathcal{C}$ with $E_n uparrow Omega$. Of course, we need this here, too! And it is satisfied as we can take the constant function $f=1$ to get $nu_1(Omega) = nu_2(Omega)$.
– p4sch
Dec 5 '18 at 8:50
By "uniqueness theorem for σ-finite measures" you mean the monotone class theorem right (although it doesn't require the existence of such a sequence)? Because that was the first approach I tried, but as I said, I couldn't prove that $mathcal{C}$ is closed under intersections.
– deque
Dec 5 '18 at 8:58
1
I mean this theorem. I called it 'uniqueness theorem for $sigma$-finite measures'.
– p4sch
Dec 5 '18 at 9:12
This theorem still requires to show that $mathcal{C}$ is closed under intersections though :/ Can you help me with that please ?
– deque
Dec 5 '18 at 11:47
|
show 3 more comments
Of course the measures involved here are finite, I edited that, sorry. The only problem remaining is that I can't prove that $mathcal{C}$ is stable under intersection. I managed to do it when the measures are of probability, but since they don't have the same total mass, I couldn't do it
– deque
Dec 5 '18 at 8:43
In the uniqueness theorem for $sigma$-finite measures you need that there exists a sequence $(E_n)$ in $mathcal{C}$ with $E_n uparrow Omega$. Of course, we need this here, too! And it is satisfied as we can take the constant function $f=1$ to get $nu_1(Omega) = nu_2(Omega)$.
– p4sch
Dec 5 '18 at 8:50
By "uniqueness theorem for σ-finite measures" you mean the monotone class theorem right (although it doesn't require the existence of such a sequence)? Because that was the first approach I tried, but as I said, I couldn't prove that $mathcal{C}$ is closed under intersections.
– deque
Dec 5 '18 at 8:58
1
I mean this theorem. I called it 'uniqueness theorem for $sigma$-finite measures'.
– p4sch
Dec 5 '18 at 9:12
This theorem still requires to show that $mathcal{C}$ is closed under intersections though :/ Can you help me with that please ?
– deque
Dec 5 '18 at 11:47
Of course the measures involved here are finite, I edited that, sorry. The only problem remaining is that I can't prove that $mathcal{C}$ is stable under intersection. I managed to do it when the measures are of probability, but since they don't have the same total mass, I couldn't do it
– deque
Dec 5 '18 at 8:43
Of course the measures involved here are finite, I edited that, sorry. The only problem remaining is that I can't prove that $mathcal{C}$ is stable under intersection. I managed to do it when the measures are of probability, but since they don't have the same total mass, I couldn't do it
– deque
Dec 5 '18 at 8:43
In the uniqueness theorem for $sigma$-finite measures you need that there exists a sequence $(E_n)$ in $mathcal{C}$ with $E_n uparrow Omega$. Of course, we need this here, too! And it is satisfied as we can take the constant function $f=1$ to get $nu_1(Omega) = nu_2(Omega)$.
– p4sch
Dec 5 '18 at 8:50
In the uniqueness theorem for $sigma$-finite measures you need that there exists a sequence $(E_n)$ in $mathcal{C}$ with $E_n uparrow Omega$. Of course, we need this here, too! And it is satisfied as we can take the constant function $f=1$ to get $nu_1(Omega) = nu_2(Omega)$.
– p4sch
Dec 5 '18 at 8:50
By "uniqueness theorem for σ-finite measures" you mean the monotone class theorem right (although it doesn't require the existence of such a sequence)? Because that was the first approach I tried, but as I said, I couldn't prove that $mathcal{C}$ is closed under intersections.
– deque
Dec 5 '18 at 8:58
By "uniqueness theorem for σ-finite measures" you mean the monotone class theorem right (although it doesn't require the existence of such a sequence)? Because that was the first approach I tried, but as I said, I couldn't prove that $mathcal{C}$ is closed under intersections.
– deque
Dec 5 '18 at 8:58
1
1
I mean this theorem. I called it 'uniqueness theorem for $sigma$-finite measures'.
– p4sch
Dec 5 '18 at 9:12
I mean this theorem. I called it 'uniqueness theorem for $sigma$-finite measures'.
– p4sch
Dec 5 '18 at 9:12
This theorem still requires to show that $mathcal{C}$ is closed under intersections though :/ Can you help me with that please ?
– deque
Dec 5 '18 at 11:47
This theorem still requires to show that $mathcal{C}$ is closed under intersections though :/ Can you help me with that please ?
– deque
Dec 5 '18 at 11:47
|
show 3 more comments
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Do you know the Riesz–Markov–Kakutani representation theorem, stating that $mathcal M(X)simeq C(X)^*$?
– Federico
Dec 4 '18 at 21:45
Nope, that seems out of my reach for now. Plus the material I'm following didn't even feel the need to highlight the equality, so I thought it was going to be similar to the positive case.
– deque
Dec 4 '18 at 21:58