Do we have $|F|_{TV([a,b])}=lim_{epsilonto 0+}|F|_{TV([a+epsilon,b])}$ if $F:[a,b]tomathbb{R}$ is continuous?
$begingroup$
Given any interval ${[a,b]}$, define the total variation ${|F|_{TV([a,b])}}$ of ${F}$ on ${[a,b]}$ as
$$
displaystyle |F|_{TV([a,b])} := sup_{a leq x_0 < ldots < x_n leq b} sum_{i=1}^n |F(x_i) - F(x_{i+1})|.
$$
Let $F:[a,b]tomathbb{R}$ be a continuous function.
Can one conclude that $$|F|_{TV([a,b])}=lim_{epsilonto 0+}|F|_{TV([a+epsilon,b])}?$$
If $F$ is absolutely continuous than this can by done by noting that
$$
|F|_{TV([c,d])}=int_c^d|F'(x)| dx.
$$
What can one say in the general case?
real-analysis bounded-variation
$endgroup$
add a comment |
$begingroup$
Given any interval ${[a,b]}$, define the total variation ${|F|_{TV([a,b])}}$ of ${F}$ on ${[a,b]}$ as
$$
displaystyle |F|_{TV([a,b])} := sup_{a leq x_0 < ldots < x_n leq b} sum_{i=1}^n |F(x_i) - F(x_{i+1})|.
$$
Let $F:[a,b]tomathbb{R}$ be a continuous function.
Can one conclude that $$|F|_{TV([a,b])}=lim_{epsilonto 0+}|F|_{TV([a+epsilon,b])}?$$
If $F$ is absolutely continuous than this can by done by noting that
$$
|F|_{TV([c,d])}=int_c^d|F'(x)| dx.
$$
What can one say in the general case?
real-analysis bounded-variation
$endgroup$
$begingroup$
Have you tried exploring the canonical examples with infinite total variation?
$endgroup$
– Alex R.
Jan 4 '17 at 0:10
$begingroup$
@AlexR. In that case, I assume that one also has $lim_{epsilonto0+}|F|_{TV([a+epsilon,b])}=infty$ and thus the equality is also true?
$endgroup$
– Jack
Jan 4 '17 at 0:13
add a comment |
$begingroup$
Given any interval ${[a,b]}$, define the total variation ${|F|_{TV([a,b])}}$ of ${F}$ on ${[a,b]}$ as
$$
displaystyle |F|_{TV([a,b])} := sup_{a leq x_0 < ldots < x_n leq b} sum_{i=1}^n |F(x_i) - F(x_{i+1})|.
$$
Let $F:[a,b]tomathbb{R}$ be a continuous function.
Can one conclude that $$|F|_{TV([a,b])}=lim_{epsilonto 0+}|F|_{TV([a+epsilon,b])}?$$
If $F$ is absolutely continuous than this can by done by noting that
$$
|F|_{TV([c,d])}=int_c^d|F'(x)| dx.
$$
What can one say in the general case?
real-analysis bounded-variation
$endgroup$
Given any interval ${[a,b]}$, define the total variation ${|F|_{TV([a,b])}}$ of ${F}$ on ${[a,b]}$ as
$$
displaystyle |F|_{TV([a,b])} := sup_{a leq x_0 < ldots < x_n leq b} sum_{i=1}^n |F(x_i) - F(x_{i+1})|.
$$
Let $F:[a,b]tomathbb{R}$ be a continuous function.
Can one conclude that $$|F|_{TV([a,b])}=lim_{epsilonto 0+}|F|_{TV([a+epsilon,b])}?$$
If $F$ is absolutely continuous than this can by done by noting that
$$
|F|_{TV([c,d])}=int_c^d|F'(x)| dx.
$$
What can one say in the general case?
real-analysis bounded-variation
real-analysis bounded-variation
asked Jan 3 '17 at 23:56
JackJack
27.4k1782200
27.4k1782200
$begingroup$
Have you tried exploring the canonical examples with infinite total variation?
$endgroup$
– Alex R.
Jan 4 '17 at 0:10
$begingroup$
@AlexR. In that case, I assume that one also has $lim_{epsilonto0+}|F|_{TV([a+epsilon,b])}=infty$ and thus the equality is also true?
$endgroup$
– Jack
Jan 4 '17 at 0:13
add a comment |
$begingroup$
Have you tried exploring the canonical examples with infinite total variation?
$endgroup$
– Alex R.
Jan 4 '17 at 0:10
$begingroup$
@AlexR. In that case, I assume that one also has $lim_{epsilonto0+}|F|_{TV([a+epsilon,b])}=infty$ and thus the equality is also true?
$endgroup$
– Jack
Jan 4 '17 at 0:13
$begingroup$
Have you tried exploring the canonical examples with infinite total variation?
$endgroup$
– Alex R.
Jan 4 '17 at 0:10
$begingroup$
Have you tried exploring the canonical examples with infinite total variation?
$endgroup$
– Alex R.
Jan 4 '17 at 0:10
$begingroup$
@AlexR. In that case, I assume that one also has $lim_{epsilonto0+}|F|_{TV([a+epsilon,b])}=infty$ and thus the equality is also true?
$endgroup$
– Jack
Jan 4 '17 at 0:13
$begingroup$
@AlexR. In that case, I assume that one also has $lim_{epsilonto0+}|F|_{TV([a+epsilon,b])}=infty$ and thus the equality is also true?
$endgroup$
– Jack
Jan 4 '17 at 0:13
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
I decided to make my comment an answer since it appears to be long.
Observe that $leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}leleftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}$ since if $P$ is a partition of $[a+epsilon,b]$
then
$$sum_{i=1}^{n}leftlvert F(x_{i})-F(x_{i-1})rightrvert leleftlvert F(a+epsilon)-F(a)rightrvert+sum_{i=1}^{n}leftlvert F(x_{i})-F(x_{i-1})rightrvertleleftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}$$
Taking a supremum over partitions gives the described result. Thus, taking a limit in $epsilon$ we obtain
$$limsup_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}leleftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}$$
Observe that through a similar proof we may obtain:
$$leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon_{1},b])}leleftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon_{2},b])}$$
when $epsilon_{1}>epsilon_{2}$. Now take a partition $P=left{a=x_{0}<ldots<x_{n}=bright}$. By continuity of $F$ at $x=a$ we can find $delta>0$ so that $leftlvert F(x)-F(a)rightrvert<eta$ if $leftlvert x-arightrvert<delta$. By perhaps refining our partition and increasing the variation we may assume $leftlvert x_{1}-arightrvert<delta$. So:
$$sum_{i=1}^{n}leftlvert F(x_{i})-F(x_{i-1})rightrvertleeta+leftlvertleftlvert Frightrvertrightrvert_{TV([x_{1},b])}leeta+liminf_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}$$
So $$leftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}leeta+liminf_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}$$
So $$leftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}leliminf_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}$$
Note that if you remove continuity of the function at $a$ then the limit is not true. For example, consider $f:[0,1]tomathbb{R}$ defined by $f(x)=chi_{left{0right}}(x)$. Then $leftlvertleftlvert frightrvertrightrvert_{TV([epsilon,1])}=0$ for $epsilon>0$ but $leftlvertleftlvert frightrvertrightvert_{TV([0,1])}=1$.
$endgroup$
add a comment |
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$begingroup$
I decided to make my comment an answer since it appears to be long.
Observe that $leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}leleftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}$ since if $P$ is a partition of $[a+epsilon,b]$
then
$$sum_{i=1}^{n}leftlvert F(x_{i})-F(x_{i-1})rightrvert leleftlvert F(a+epsilon)-F(a)rightrvert+sum_{i=1}^{n}leftlvert F(x_{i})-F(x_{i-1})rightrvertleleftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}$$
Taking a supremum over partitions gives the described result. Thus, taking a limit in $epsilon$ we obtain
$$limsup_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}leleftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}$$
Observe that through a similar proof we may obtain:
$$leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon_{1},b])}leleftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon_{2},b])}$$
when $epsilon_{1}>epsilon_{2}$. Now take a partition $P=left{a=x_{0}<ldots<x_{n}=bright}$. By continuity of $F$ at $x=a$ we can find $delta>0$ so that $leftlvert F(x)-F(a)rightrvert<eta$ if $leftlvert x-arightrvert<delta$. By perhaps refining our partition and increasing the variation we may assume $leftlvert x_{1}-arightrvert<delta$. So:
$$sum_{i=1}^{n}leftlvert F(x_{i})-F(x_{i-1})rightrvertleeta+leftlvertleftlvert Frightrvertrightrvert_{TV([x_{1},b])}leeta+liminf_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}$$
So $$leftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}leeta+liminf_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}$$
So $$leftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}leliminf_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}$$
Note that if you remove continuity of the function at $a$ then the limit is not true. For example, consider $f:[0,1]tomathbb{R}$ defined by $f(x)=chi_{left{0right}}(x)$. Then $leftlvertleftlvert frightrvertrightrvert_{TV([epsilon,1])}=0$ for $epsilon>0$ but $leftlvertleftlvert frightrvertrightvert_{TV([0,1])}=1$.
$endgroup$
add a comment |
$begingroup$
I decided to make my comment an answer since it appears to be long.
Observe that $leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}leleftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}$ since if $P$ is a partition of $[a+epsilon,b]$
then
$$sum_{i=1}^{n}leftlvert F(x_{i})-F(x_{i-1})rightrvert leleftlvert F(a+epsilon)-F(a)rightrvert+sum_{i=1}^{n}leftlvert F(x_{i})-F(x_{i-1})rightrvertleleftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}$$
Taking a supremum over partitions gives the described result. Thus, taking a limit in $epsilon$ we obtain
$$limsup_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}leleftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}$$
Observe that through a similar proof we may obtain:
$$leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon_{1},b])}leleftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon_{2},b])}$$
when $epsilon_{1}>epsilon_{2}$. Now take a partition $P=left{a=x_{0}<ldots<x_{n}=bright}$. By continuity of $F$ at $x=a$ we can find $delta>0$ so that $leftlvert F(x)-F(a)rightrvert<eta$ if $leftlvert x-arightrvert<delta$. By perhaps refining our partition and increasing the variation we may assume $leftlvert x_{1}-arightrvert<delta$. So:
$$sum_{i=1}^{n}leftlvert F(x_{i})-F(x_{i-1})rightrvertleeta+leftlvertleftlvert Frightrvertrightrvert_{TV([x_{1},b])}leeta+liminf_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}$$
So $$leftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}leeta+liminf_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}$$
So $$leftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}leliminf_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}$$
Note that if you remove continuity of the function at $a$ then the limit is not true. For example, consider $f:[0,1]tomathbb{R}$ defined by $f(x)=chi_{left{0right}}(x)$. Then $leftlvertleftlvert frightrvertrightrvert_{TV([epsilon,1])}=0$ for $epsilon>0$ but $leftlvertleftlvert frightrvertrightvert_{TV([0,1])}=1$.
$endgroup$
add a comment |
$begingroup$
I decided to make my comment an answer since it appears to be long.
Observe that $leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}leleftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}$ since if $P$ is a partition of $[a+epsilon,b]$
then
$$sum_{i=1}^{n}leftlvert F(x_{i})-F(x_{i-1})rightrvert leleftlvert F(a+epsilon)-F(a)rightrvert+sum_{i=1}^{n}leftlvert F(x_{i})-F(x_{i-1})rightrvertleleftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}$$
Taking a supremum over partitions gives the described result. Thus, taking a limit in $epsilon$ we obtain
$$limsup_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}leleftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}$$
Observe that through a similar proof we may obtain:
$$leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon_{1},b])}leleftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon_{2},b])}$$
when $epsilon_{1}>epsilon_{2}$. Now take a partition $P=left{a=x_{0}<ldots<x_{n}=bright}$. By continuity of $F$ at $x=a$ we can find $delta>0$ so that $leftlvert F(x)-F(a)rightrvert<eta$ if $leftlvert x-arightrvert<delta$. By perhaps refining our partition and increasing the variation we may assume $leftlvert x_{1}-arightrvert<delta$. So:
$$sum_{i=1}^{n}leftlvert F(x_{i})-F(x_{i-1})rightrvertleeta+leftlvertleftlvert Frightrvertrightrvert_{TV([x_{1},b])}leeta+liminf_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}$$
So $$leftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}leeta+liminf_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}$$
So $$leftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}leliminf_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}$$
Note that if you remove continuity of the function at $a$ then the limit is not true. For example, consider $f:[0,1]tomathbb{R}$ defined by $f(x)=chi_{left{0right}}(x)$. Then $leftlvertleftlvert frightrvertrightrvert_{TV([epsilon,1])}=0$ for $epsilon>0$ but $leftlvertleftlvert frightrvertrightvert_{TV([0,1])}=1$.
$endgroup$
I decided to make my comment an answer since it appears to be long.
Observe that $leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}leleftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}$ since if $P$ is a partition of $[a+epsilon,b]$
then
$$sum_{i=1}^{n}leftlvert F(x_{i})-F(x_{i-1})rightrvert leleftlvert F(a+epsilon)-F(a)rightrvert+sum_{i=1}^{n}leftlvert F(x_{i})-F(x_{i-1})rightrvertleleftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}$$
Taking a supremum over partitions gives the described result. Thus, taking a limit in $epsilon$ we obtain
$$limsup_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}leleftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}$$
Observe that through a similar proof we may obtain:
$$leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon_{1},b])}leleftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon_{2},b])}$$
when $epsilon_{1}>epsilon_{2}$. Now take a partition $P=left{a=x_{0}<ldots<x_{n}=bright}$. By continuity of $F$ at $x=a$ we can find $delta>0$ so that $leftlvert F(x)-F(a)rightrvert<eta$ if $leftlvert x-arightrvert<delta$. By perhaps refining our partition and increasing the variation we may assume $leftlvert x_{1}-arightrvert<delta$. So:
$$sum_{i=1}^{n}leftlvert F(x_{i})-F(x_{i-1})rightrvertleeta+leftlvertleftlvert Frightrvertrightrvert_{TV([x_{1},b])}leeta+liminf_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}$$
So $$leftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}leeta+liminf_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}$$
So $$leftlvertleftlvert Frightrvertrightrvert_{TV([a,b])}leliminf_{epsilonto0^{+}}leftlvertleftlvert Frightrvertrightrvert_{TV([a+epsilon,b])}$$
Note that if you remove continuity of the function at $a$ then the limit is not true. For example, consider $f:[0,1]tomathbb{R}$ defined by $f(x)=chi_{left{0right}}(x)$. Then $leftlvertleftlvert frightrvertrightrvert_{TV([epsilon,1])}=0$ for $epsilon>0$ but $leftlvertleftlvert frightrvertrightvert_{TV([0,1])}=1$.
edited Dec 22 '18 at 16:50
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5 revs, 2 users 94%
user71352
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$begingroup$
Have you tried exploring the canonical examples with infinite total variation?
$endgroup$
– Alex R.
Jan 4 '17 at 0:10
$begingroup$
@AlexR. In that case, I assume that one also has $lim_{epsilonto0+}|F|_{TV([a+epsilon,b])}=infty$ and thus the equality is also true?
$endgroup$
– Jack
Jan 4 '17 at 0:13