Assuming that $f$ is a bounded linear function on $X$, compute $|f|$.
$begingroup$
Let $X$ be a finite dimensional linear space and let ${e_i}^{n}_{i=1}$ be a basis. Hence, there exists unique scalars ${alpha_i}^{n}_{i=1}$ such that
begin{align} x=sum^{n}_{i=1}alpha_i e_i.end{align}
Now, we define $|cdot|$ by begin{align} |x|=left(sum^{n}_{i=1}|alpha_i |^2right)^{1/2}.end{align}
Assuming that $f$ is a bounded linear function on $X$, I want compute $|f|$.
MY TRIAL
Since $f$ is a bounded linear function on $X$, then there exists $Kgeq 0$ such that
begin{align} |f(x)|leq K|x|,;;forall;xin X.end{align}
So, taking $sup$ over $|x|leq 1,$ we get
begin{align} |f|=suplimits_{|x|leq 1}|f(x)|leq K,;;forall;xin X.end{align}
I might be wrong. Kindly check if I'm wrong or right. If I'm wrong, can you give me an idea of what to do?
functional-analysis normed-spaces
$endgroup$
add a comment |
$begingroup$
Let $X$ be a finite dimensional linear space and let ${e_i}^{n}_{i=1}$ be a basis. Hence, there exists unique scalars ${alpha_i}^{n}_{i=1}$ such that
begin{align} x=sum^{n}_{i=1}alpha_i e_i.end{align}
Now, we define $|cdot|$ by begin{align} |x|=left(sum^{n}_{i=1}|alpha_i |^2right)^{1/2}.end{align}
Assuming that $f$ is a bounded linear function on $X$, I want compute $|f|$.
MY TRIAL
Since $f$ is a bounded linear function on $X$, then there exists $Kgeq 0$ such that
begin{align} |f(x)|leq K|x|,;;forall;xin X.end{align}
So, taking $sup$ over $|x|leq 1,$ we get
begin{align} |f|=suplimits_{|x|leq 1}|f(x)|leq K,;;forall;xin X.end{align}
I might be wrong. Kindly check if I'm wrong or right. If I'm wrong, can you give me an idea of what to do?
functional-analysis normed-spaces
$endgroup$
$begingroup$
I don't use the same definition of a bounded function... Are you sure of yours?
$endgroup$
– Damien
Dec 27 '18 at 8:22
$begingroup$
@Damien: Yes, you can also check en.wikipedia.org/wiki/Bounded_operator, to verify!
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:24
$begingroup$
@Damien: Which do you use?
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:24
$begingroup$
It appears that the definition of a bounded linear function is different from the definition of a bounded function. I did not know. Yo are right. Sorry
$endgroup$
– Damien
Dec 27 '18 at 8:35
$begingroup$
@Damien: No problems! We all learn!
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:51
add a comment |
$begingroup$
Let $X$ be a finite dimensional linear space and let ${e_i}^{n}_{i=1}$ be a basis. Hence, there exists unique scalars ${alpha_i}^{n}_{i=1}$ such that
begin{align} x=sum^{n}_{i=1}alpha_i e_i.end{align}
Now, we define $|cdot|$ by begin{align} |x|=left(sum^{n}_{i=1}|alpha_i |^2right)^{1/2}.end{align}
Assuming that $f$ is a bounded linear function on $X$, I want compute $|f|$.
MY TRIAL
Since $f$ is a bounded linear function on $X$, then there exists $Kgeq 0$ such that
begin{align} |f(x)|leq K|x|,;;forall;xin X.end{align}
So, taking $sup$ over $|x|leq 1,$ we get
begin{align} |f|=suplimits_{|x|leq 1}|f(x)|leq K,;;forall;xin X.end{align}
I might be wrong. Kindly check if I'm wrong or right. If I'm wrong, can you give me an idea of what to do?
functional-analysis normed-spaces
$endgroup$
Let $X$ be a finite dimensional linear space and let ${e_i}^{n}_{i=1}$ be a basis. Hence, there exists unique scalars ${alpha_i}^{n}_{i=1}$ such that
begin{align} x=sum^{n}_{i=1}alpha_i e_i.end{align}
Now, we define $|cdot|$ by begin{align} |x|=left(sum^{n}_{i=1}|alpha_i |^2right)^{1/2}.end{align}
Assuming that $f$ is a bounded linear function on $X$, I want compute $|f|$.
MY TRIAL
Since $f$ is a bounded linear function on $X$, then there exists $Kgeq 0$ such that
begin{align} |f(x)|leq K|x|,;;forall;xin X.end{align}
So, taking $sup$ over $|x|leq 1,$ we get
begin{align} |f|=suplimits_{|x|leq 1}|f(x)|leq K,;;forall;xin X.end{align}
I might be wrong. Kindly check if I'm wrong or right. If I'm wrong, can you give me an idea of what to do?
functional-analysis normed-spaces
functional-analysis normed-spaces
edited Dec 27 '18 at 8:31
Omojola Micheal
asked Dec 27 '18 at 8:12
Omojola MichealOmojola Micheal
1,875324
1,875324
$begingroup$
I don't use the same definition of a bounded function... Are you sure of yours?
$endgroup$
– Damien
Dec 27 '18 at 8:22
$begingroup$
@Damien: Yes, you can also check en.wikipedia.org/wiki/Bounded_operator, to verify!
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:24
$begingroup$
@Damien: Which do you use?
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:24
$begingroup$
It appears that the definition of a bounded linear function is different from the definition of a bounded function. I did not know. Yo are right. Sorry
$endgroup$
– Damien
Dec 27 '18 at 8:35
$begingroup$
@Damien: No problems! We all learn!
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:51
add a comment |
$begingroup$
I don't use the same definition of a bounded function... Are you sure of yours?
$endgroup$
– Damien
Dec 27 '18 at 8:22
$begingroup$
@Damien: Yes, you can also check en.wikipedia.org/wiki/Bounded_operator, to verify!
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:24
$begingroup$
@Damien: Which do you use?
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:24
$begingroup$
It appears that the definition of a bounded linear function is different from the definition of a bounded function. I did not know. Yo are right. Sorry
$endgroup$
– Damien
Dec 27 '18 at 8:35
$begingroup$
@Damien: No problems! We all learn!
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:51
$begingroup$
I don't use the same definition of a bounded function... Are you sure of yours?
$endgroup$
– Damien
Dec 27 '18 at 8:22
$begingroup$
I don't use the same definition of a bounded function... Are you sure of yours?
$endgroup$
– Damien
Dec 27 '18 at 8:22
$begingroup$
@Damien: Yes, you can also check en.wikipedia.org/wiki/Bounded_operator, to verify!
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:24
$begingroup$
@Damien: Yes, you can also check en.wikipedia.org/wiki/Bounded_operator, to verify!
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:24
$begingroup$
@Damien: Which do you use?
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:24
$begingroup$
@Damien: Which do you use?
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:24
$begingroup$
It appears that the definition of a bounded linear function is different from the definition of a bounded function. I did not know. Yo are right. Sorry
$endgroup$
– Damien
Dec 27 '18 at 8:35
$begingroup$
It appears that the definition of a bounded linear function is different from the definition of a bounded function. I did not know. Yo are right. Sorry
$endgroup$
– Damien
Dec 27 '18 at 8:35
$begingroup$
@Damien: No problems! We all learn!
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:51
$begingroup$
@Damien: No problems! We all learn!
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:51
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
$|f|=sup {frac {|f(sum a_ie_i)|} {|sum a_ie_i|}}leqsqrt {sum |f(e_i)|^{2}}$ by C-S inequality. The exact value of $|f|$ is not easy to write and it is not $sqrt {sum |f(e_i)|^{2}}$ in general. For example, User Damien has given example in a comment in which the norm is not equal to $sqrt {sum |f(e_i)|^{2}}$
$endgroup$
$begingroup$
Sorry, I find this result very strange. For example, consider $n=2$, $f(e_1)=e_1$, $f(e_2)=2e_2$
$endgroup$
– Damien
Dec 27 '18 at 8:52
1
$begingroup$
@Damien I thought $f$ was a continuous linear functional but I now realize that it is a linear map of $X$ into $X$. The exact value of $|f|$ is hard to write. From what the OP has written it is not clear if he is trying to write an explicitly formula for $|f|$. Thanks for your comment.
$endgroup$
– Kavi Rama Murthy
Dec 27 '18 at 9:09
add a comment |
$begingroup$
I guess it should be:
begin{align} |f|&=suplimits_{xneq 0}dfrac{|f(x)|}{|x|}\&=suplimits_{alpha_ineq 0}dfrac{left|f(sum^{n}_{i=1}alpha_i e_i)right|}{left(sum^{n}_{i=1}|alpha_i |^2right)^{1/2}}\&=suplimits_{alpha_ineq 0}dfrac{left|sum^{n}_{i=1}alpha_i f(e_i)right|}{left(sum^{n}_{i=1}|alpha_i |^2right)^{1/2}}.end{align}
$endgroup$
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
$|f|=sup {frac {|f(sum a_ie_i)|} {|sum a_ie_i|}}leqsqrt {sum |f(e_i)|^{2}}$ by C-S inequality. The exact value of $|f|$ is not easy to write and it is not $sqrt {sum |f(e_i)|^{2}}$ in general. For example, User Damien has given example in a comment in which the norm is not equal to $sqrt {sum |f(e_i)|^{2}}$
$endgroup$
$begingroup$
Sorry, I find this result very strange. For example, consider $n=2$, $f(e_1)=e_1$, $f(e_2)=2e_2$
$endgroup$
– Damien
Dec 27 '18 at 8:52
1
$begingroup$
@Damien I thought $f$ was a continuous linear functional but I now realize that it is a linear map of $X$ into $X$. The exact value of $|f|$ is hard to write. From what the OP has written it is not clear if he is trying to write an explicitly formula for $|f|$. Thanks for your comment.
$endgroup$
– Kavi Rama Murthy
Dec 27 '18 at 9:09
add a comment |
$begingroup$
$|f|=sup {frac {|f(sum a_ie_i)|} {|sum a_ie_i|}}leqsqrt {sum |f(e_i)|^{2}}$ by C-S inequality. The exact value of $|f|$ is not easy to write and it is not $sqrt {sum |f(e_i)|^{2}}$ in general. For example, User Damien has given example in a comment in which the norm is not equal to $sqrt {sum |f(e_i)|^{2}}$
$endgroup$
$begingroup$
Sorry, I find this result very strange. For example, consider $n=2$, $f(e_1)=e_1$, $f(e_2)=2e_2$
$endgroup$
– Damien
Dec 27 '18 at 8:52
1
$begingroup$
@Damien I thought $f$ was a continuous linear functional but I now realize that it is a linear map of $X$ into $X$. The exact value of $|f|$ is hard to write. From what the OP has written it is not clear if he is trying to write an explicitly formula for $|f|$. Thanks for your comment.
$endgroup$
– Kavi Rama Murthy
Dec 27 '18 at 9:09
add a comment |
$begingroup$
$|f|=sup {frac {|f(sum a_ie_i)|} {|sum a_ie_i|}}leqsqrt {sum |f(e_i)|^{2}}$ by C-S inequality. The exact value of $|f|$ is not easy to write and it is not $sqrt {sum |f(e_i)|^{2}}$ in general. For example, User Damien has given example in a comment in which the norm is not equal to $sqrt {sum |f(e_i)|^{2}}$
$endgroup$
$|f|=sup {frac {|f(sum a_ie_i)|} {|sum a_ie_i|}}leqsqrt {sum |f(e_i)|^{2}}$ by C-S inequality. The exact value of $|f|$ is not easy to write and it is not $sqrt {sum |f(e_i)|^{2}}$ in general. For example, User Damien has given example in a comment in which the norm is not equal to $sqrt {sum |f(e_i)|^{2}}$
edited Dec 27 '18 at 9:15
answered Dec 27 '18 at 8:20
Kavi Rama MurthyKavi Rama Murthy
60.5k42161
60.5k42161
$begingroup$
Sorry, I find this result very strange. For example, consider $n=2$, $f(e_1)=e_1$, $f(e_2)=2e_2$
$endgroup$
– Damien
Dec 27 '18 at 8:52
1
$begingroup$
@Damien I thought $f$ was a continuous linear functional but I now realize that it is a linear map of $X$ into $X$. The exact value of $|f|$ is hard to write. From what the OP has written it is not clear if he is trying to write an explicitly formula for $|f|$. Thanks for your comment.
$endgroup$
– Kavi Rama Murthy
Dec 27 '18 at 9:09
add a comment |
$begingroup$
Sorry, I find this result very strange. For example, consider $n=2$, $f(e_1)=e_1$, $f(e_2)=2e_2$
$endgroup$
– Damien
Dec 27 '18 at 8:52
1
$begingroup$
@Damien I thought $f$ was a continuous linear functional but I now realize that it is a linear map of $X$ into $X$. The exact value of $|f|$ is hard to write. From what the OP has written it is not clear if he is trying to write an explicitly formula for $|f|$. Thanks for your comment.
$endgroup$
– Kavi Rama Murthy
Dec 27 '18 at 9:09
$begingroup$
Sorry, I find this result very strange. For example, consider $n=2$, $f(e_1)=e_1$, $f(e_2)=2e_2$
$endgroup$
– Damien
Dec 27 '18 at 8:52
$begingroup$
Sorry, I find this result very strange. For example, consider $n=2$, $f(e_1)=e_1$, $f(e_2)=2e_2$
$endgroup$
– Damien
Dec 27 '18 at 8:52
1
1
$begingroup$
@Damien I thought $f$ was a continuous linear functional but I now realize that it is a linear map of $X$ into $X$. The exact value of $|f|$ is hard to write. From what the OP has written it is not clear if he is trying to write an explicitly formula for $|f|$. Thanks for your comment.
$endgroup$
– Kavi Rama Murthy
Dec 27 '18 at 9:09
$begingroup$
@Damien I thought $f$ was a continuous linear functional but I now realize that it is a linear map of $X$ into $X$. The exact value of $|f|$ is hard to write. From what the OP has written it is not clear if he is trying to write an explicitly formula for $|f|$. Thanks for your comment.
$endgroup$
– Kavi Rama Murthy
Dec 27 '18 at 9:09
add a comment |
$begingroup$
I guess it should be:
begin{align} |f|&=suplimits_{xneq 0}dfrac{|f(x)|}{|x|}\&=suplimits_{alpha_ineq 0}dfrac{left|f(sum^{n}_{i=1}alpha_i e_i)right|}{left(sum^{n}_{i=1}|alpha_i |^2right)^{1/2}}\&=suplimits_{alpha_ineq 0}dfrac{left|sum^{n}_{i=1}alpha_i f(e_i)right|}{left(sum^{n}_{i=1}|alpha_i |^2right)^{1/2}}.end{align}
$endgroup$
add a comment |
$begingroup$
I guess it should be:
begin{align} |f|&=suplimits_{xneq 0}dfrac{|f(x)|}{|x|}\&=suplimits_{alpha_ineq 0}dfrac{left|f(sum^{n}_{i=1}alpha_i e_i)right|}{left(sum^{n}_{i=1}|alpha_i |^2right)^{1/2}}\&=suplimits_{alpha_ineq 0}dfrac{left|sum^{n}_{i=1}alpha_i f(e_i)right|}{left(sum^{n}_{i=1}|alpha_i |^2right)^{1/2}}.end{align}
$endgroup$
add a comment |
$begingroup$
I guess it should be:
begin{align} |f|&=suplimits_{xneq 0}dfrac{|f(x)|}{|x|}\&=suplimits_{alpha_ineq 0}dfrac{left|f(sum^{n}_{i=1}alpha_i e_i)right|}{left(sum^{n}_{i=1}|alpha_i |^2right)^{1/2}}\&=suplimits_{alpha_ineq 0}dfrac{left|sum^{n}_{i=1}alpha_i f(e_i)right|}{left(sum^{n}_{i=1}|alpha_i |^2right)^{1/2}}.end{align}
$endgroup$
I guess it should be:
begin{align} |f|&=suplimits_{xneq 0}dfrac{|f(x)|}{|x|}\&=suplimits_{alpha_ineq 0}dfrac{left|f(sum^{n}_{i=1}alpha_i e_i)right|}{left(sum^{n}_{i=1}|alpha_i |^2right)^{1/2}}\&=suplimits_{alpha_ineq 0}dfrac{left|sum^{n}_{i=1}alpha_i f(e_i)right|}{left(sum^{n}_{i=1}|alpha_i |^2right)^{1/2}}.end{align}
answered Dec 27 '18 at 14:39
Omojola MichealOmojola Micheal
1,875324
1,875324
add a comment |
add a comment |
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$begingroup$
I don't use the same definition of a bounded function... Are you sure of yours?
$endgroup$
– Damien
Dec 27 '18 at 8:22
$begingroup$
@Damien: Yes, you can also check en.wikipedia.org/wiki/Bounded_operator, to verify!
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:24
$begingroup$
@Damien: Which do you use?
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:24
$begingroup$
It appears that the definition of a bounded linear function is different from the definition of a bounded function. I did not know. Yo are right. Sorry
$endgroup$
– Damien
Dec 27 '18 at 8:35
$begingroup$
@Damien: No problems! We all learn!
$endgroup$
– Omojola Micheal
Dec 27 '18 at 8:51