The Picard-Lindelöf theorem on Wikipedia
$begingroup$
On the Wikipedia entry of Picard-Lindelöf theorem for the local existence and uniqueness of ODE's, there is a section on the optimization of the solution's interval.
There is a lemma used in this section which says
$$
|| Gamma^m varphi_1 - Gamma^m varphi_2 || leq dfrac{L^m alpha^m}{m!} ||varphi_1 - varphi_2 ||
$$
Please see the article for the definition of symbols above.
I can not get the $m!$ term in the denominator and believe that the lemma is wrong. Am I right?
ordinary-differential-equations
$endgroup$
add a comment |
$begingroup$
On the Wikipedia entry of Picard-Lindelöf theorem for the local existence and uniqueness of ODE's, there is a section on the optimization of the solution's interval.
There is a lemma used in this section which says
$$
|| Gamma^m varphi_1 - Gamma^m varphi_2 || leq dfrac{L^m alpha^m}{m!} ||varphi_1 - varphi_2 ||
$$
Please see the article for the definition of symbols above.
I can not get the $m!$ term in the denominator and believe that the lemma is wrong. Am I right?
ordinary-differential-equations
$endgroup$
$begingroup$
There is also a proof given in the section you cite. Have you read it? Where do you disagree?
$endgroup$
– martini
Jul 15 '14 at 10:03
$begingroup$
m! term should not appear in the denominator.
$endgroup$
– pitonist
Jul 15 '14 at 10:07
1
$begingroup$
I think one should replace the claim $$ |Gamma^m phi_1 - Gamma^m phi_2 | le frac{ L^m alpha^m}{m!}|phi_1-phi_2| $$ by $$ |Gamma^m phi_1(s) - Gamma^m phi_2(s) | le frac{ L^m}{m!}(s-t_0)^m|phi_1-phi_2| quad forall sin(t_0,t_0+a). $$ Very sloppy proof there.
$endgroup$
– daw
Jul 15 '14 at 10:20
add a comment |
$begingroup$
On the Wikipedia entry of Picard-Lindelöf theorem for the local existence and uniqueness of ODE's, there is a section on the optimization of the solution's interval.
There is a lemma used in this section which says
$$
|| Gamma^m varphi_1 - Gamma^m varphi_2 || leq dfrac{L^m alpha^m}{m!} ||varphi_1 - varphi_2 ||
$$
Please see the article for the definition of symbols above.
I can not get the $m!$ term in the denominator and believe that the lemma is wrong. Am I right?
ordinary-differential-equations
$endgroup$
On the Wikipedia entry of Picard-Lindelöf theorem for the local existence and uniqueness of ODE's, there is a section on the optimization of the solution's interval.
There is a lemma used in this section which says
$$
|| Gamma^m varphi_1 - Gamma^m varphi_2 || leq dfrac{L^m alpha^m}{m!} ||varphi_1 - varphi_2 ||
$$
Please see the article for the definition of symbols above.
I can not get the $m!$ term in the denominator and believe that the lemma is wrong. Am I right?
ordinary-differential-equations
ordinary-differential-equations
edited Jul 30 '15 at 11:49
Jyrki Lahtonen
110k13171386
110k13171386
asked Jul 15 '14 at 9:56
pitonistpitonist
7918
7918
$begingroup$
There is also a proof given in the section you cite. Have you read it? Where do you disagree?
$endgroup$
– martini
Jul 15 '14 at 10:03
$begingroup$
m! term should not appear in the denominator.
$endgroup$
– pitonist
Jul 15 '14 at 10:07
1
$begingroup$
I think one should replace the claim $$ |Gamma^m phi_1 - Gamma^m phi_2 | le frac{ L^m alpha^m}{m!}|phi_1-phi_2| $$ by $$ |Gamma^m phi_1(s) - Gamma^m phi_2(s) | le frac{ L^m}{m!}(s-t_0)^m|phi_1-phi_2| quad forall sin(t_0,t_0+a). $$ Very sloppy proof there.
$endgroup$
– daw
Jul 15 '14 at 10:20
add a comment |
$begingroup$
There is also a proof given in the section you cite. Have you read it? Where do you disagree?
$endgroup$
– martini
Jul 15 '14 at 10:03
$begingroup$
m! term should not appear in the denominator.
$endgroup$
– pitonist
Jul 15 '14 at 10:07
1
$begingroup$
I think one should replace the claim $$ |Gamma^m phi_1 - Gamma^m phi_2 | le frac{ L^m alpha^m}{m!}|phi_1-phi_2| $$ by $$ |Gamma^m phi_1(s) - Gamma^m phi_2(s) | le frac{ L^m}{m!}(s-t_0)^m|phi_1-phi_2| quad forall sin(t_0,t_0+a). $$ Very sloppy proof there.
$endgroup$
– daw
Jul 15 '14 at 10:20
$begingroup$
There is also a proof given in the section you cite. Have you read it? Where do you disagree?
$endgroup$
– martini
Jul 15 '14 at 10:03
$begingroup$
There is also a proof given in the section you cite. Have you read it? Where do you disagree?
$endgroup$
– martini
Jul 15 '14 at 10:03
$begingroup$
m! term should not appear in the denominator.
$endgroup$
– pitonist
Jul 15 '14 at 10:07
$begingroup$
m! term should not appear in the denominator.
$endgroup$
– pitonist
Jul 15 '14 at 10:07
1
1
$begingroup$
I think one should replace the claim $$ |Gamma^m phi_1 - Gamma^m phi_2 | le frac{ L^m alpha^m}{m!}|phi_1-phi_2| $$ by $$ |Gamma^m phi_1(s) - Gamma^m phi_2(s) | le frac{ L^m}{m!}(s-t_0)^m|phi_1-phi_2| quad forall sin(t_0,t_0+a). $$ Very sloppy proof there.
$endgroup$
– daw
Jul 15 '14 at 10:20
$begingroup$
I think one should replace the claim $$ |Gamma^m phi_1 - Gamma^m phi_2 | le frac{ L^m alpha^m}{m!}|phi_1-phi_2| $$ by $$ |Gamma^m phi_1(s) - Gamma^m phi_2(s) | le frac{ L^m}{m!}(s-t_0)^m|phi_1-phi_2| quad forall sin(t_0,t_0+a). $$ Very sloppy proof there.
$endgroup$
– daw
Jul 15 '14 at 10:20
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
First, let's prove this:
$$
|| Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || leq dfrac{L^{m} |t-t_0|^{m}}{m!} ||varphi_1 - varphi_2 ||, forall t in I_a
$$
With $m = 1$, is trivial. If the induction hypothesis is:
$$|| Gamma^{m-1} varphi_1(t) - Gamma^{m-1} varphi_2(t) || leq dfrac{L^{m-1} |t-t_0|^{m-1}}{(m-1)!} ||varphi_1 - varphi_2 ||, forall t in I_a
$$
Then $forall t in I_a$:
begin{align}
|| Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || = left|left| GammaGamma^{m-1} varphi_1(t) - Gamma Gamma^{m-1} varphi_2(t) right|right| &= left|left| int_{t_0}^{t} fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) ds right|right| \
&leq left| int_{t_0}^{t} left|left| fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) right|right| ds right|
end{align}
But $f$ is Lipschitz in $I_a times B_b$ in the second variable with $L$ as Lipschitz constant. Then:
begin{align}
left| int_{t_0}^{t} left|left| fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) right|right| ds right|
&leq
L left| int_{t_0}^{t} left|left| Gamma^{m-1} varphi_1(s) - Gamma^{m-1} varphi_2(s) right|right| ds right| \
end{align}
And by induction hypothesis:
begin{align}
left| int_{t_0}^{t} left|left| fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) right|right| ds right|
&leq
L left| int_{t_0}^{t} dfrac{L^{m-1} |s-t_0|^{m-1}}{(m-1)!} ||varphi_1 - varphi_2 || ds right| \
&leq
dfrac{L^m}{(m-1)!} left| int_{t_0}^{t} |s-t_0|^{m-1} ||varphi_1 - varphi_2 || ds right| \
&leq
frac{L^m|t-t_0|^m}{m!} ||varphi_1 - varphi_2 || \
end{align}
Then the previous proposition is right, now:
begin{align}
|| Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || leq dfrac{L^{m} |t-t_0|^{m}}{m!} ||varphi_1 - varphi_2 || leq dfrac{L^{m} a^{m}}{m!} ||varphi_1 - varphi_2 ||, forall t in I_a
end{align}
Then:
begin{align}
textrm{sup} left{ || Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || : t in I_a right} &leq dfrac{L^{m} a^{m}}{m!} ||varphi_1 - varphi_2 ||, forall t in I_a \
|| Gamma^{m} varphi_1 - Gamma^{m} varphi_2 || &leq dfrac{L^{m} a^{m}}{m!} ||varphi_1 - varphi_2 ||
end{align}
$endgroup$
add a comment |
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$begingroup$
First, let's prove this:
$$
|| Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || leq dfrac{L^{m} |t-t_0|^{m}}{m!} ||varphi_1 - varphi_2 ||, forall t in I_a
$$
With $m = 1$, is trivial. If the induction hypothesis is:
$$|| Gamma^{m-1} varphi_1(t) - Gamma^{m-1} varphi_2(t) || leq dfrac{L^{m-1} |t-t_0|^{m-1}}{(m-1)!} ||varphi_1 - varphi_2 ||, forall t in I_a
$$
Then $forall t in I_a$:
begin{align}
|| Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || = left|left| GammaGamma^{m-1} varphi_1(t) - Gamma Gamma^{m-1} varphi_2(t) right|right| &= left|left| int_{t_0}^{t} fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) ds right|right| \
&leq left| int_{t_0}^{t} left|left| fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) right|right| ds right|
end{align}
But $f$ is Lipschitz in $I_a times B_b$ in the second variable with $L$ as Lipschitz constant. Then:
begin{align}
left| int_{t_0}^{t} left|left| fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) right|right| ds right|
&leq
L left| int_{t_0}^{t} left|left| Gamma^{m-1} varphi_1(s) - Gamma^{m-1} varphi_2(s) right|right| ds right| \
end{align}
And by induction hypothesis:
begin{align}
left| int_{t_0}^{t} left|left| fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) right|right| ds right|
&leq
L left| int_{t_0}^{t} dfrac{L^{m-1} |s-t_0|^{m-1}}{(m-1)!} ||varphi_1 - varphi_2 || ds right| \
&leq
dfrac{L^m}{(m-1)!} left| int_{t_0}^{t} |s-t_0|^{m-1} ||varphi_1 - varphi_2 || ds right| \
&leq
frac{L^m|t-t_0|^m}{m!} ||varphi_1 - varphi_2 || \
end{align}
Then the previous proposition is right, now:
begin{align}
|| Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || leq dfrac{L^{m} |t-t_0|^{m}}{m!} ||varphi_1 - varphi_2 || leq dfrac{L^{m} a^{m}}{m!} ||varphi_1 - varphi_2 ||, forall t in I_a
end{align}
Then:
begin{align}
textrm{sup} left{ || Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || : t in I_a right} &leq dfrac{L^{m} a^{m}}{m!} ||varphi_1 - varphi_2 ||, forall t in I_a \
|| Gamma^{m} varphi_1 - Gamma^{m} varphi_2 || &leq dfrac{L^{m} a^{m}}{m!} ||varphi_1 - varphi_2 ||
end{align}
$endgroup$
add a comment |
$begingroup$
First, let's prove this:
$$
|| Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || leq dfrac{L^{m} |t-t_0|^{m}}{m!} ||varphi_1 - varphi_2 ||, forall t in I_a
$$
With $m = 1$, is trivial. If the induction hypothesis is:
$$|| Gamma^{m-1} varphi_1(t) - Gamma^{m-1} varphi_2(t) || leq dfrac{L^{m-1} |t-t_0|^{m-1}}{(m-1)!} ||varphi_1 - varphi_2 ||, forall t in I_a
$$
Then $forall t in I_a$:
begin{align}
|| Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || = left|left| GammaGamma^{m-1} varphi_1(t) - Gamma Gamma^{m-1} varphi_2(t) right|right| &= left|left| int_{t_0}^{t} fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) ds right|right| \
&leq left| int_{t_0}^{t} left|left| fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) right|right| ds right|
end{align}
But $f$ is Lipschitz in $I_a times B_b$ in the second variable with $L$ as Lipschitz constant. Then:
begin{align}
left| int_{t_0}^{t} left|left| fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) right|right| ds right|
&leq
L left| int_{t_0}^{t} left|left| Gamma^{m-1} varphi_1(s) - Gamma^{m-1} varphi_2(s) right|right| ds right| \
end{align}
And by induction hypothesis:
begin{align}
left| int_{t_0}^{t} left|left| fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) right|right| ds right|
&leq
L left| int_{t_0}^{t} dfrac{L^{m-1} |s-t_0|^{m-1}}{(m-1)!} ||varphi_1 - varphi_2 || ds right| \
&leq
dfrac{L^m}{(m-1)!} left| int_{t_0}^{t} |s-t_0|^{m-1} ||varphi_1 - varphi_2 || ds right| \
&leq
frac{L^m|t-t_0|^m}{m!} ||varphi_1 - varphi_2 || \
end{align}
Then the previous proposition is right, now:
begin{align}
|| Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || leq dfrac{L^{m} |t-t_0|^{m}}{m!} ||varphi_1 - varphi_2 || leq dfrac{L^{m} a^{m}}{m!} ||varphi_1 - varphi_2 ||, forall t in I_a
end{align}
Then:
begin{align}
textrm{sup} left{ || Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || : t in I_a right} &leq dfrac{L^{m} a^{m}}{m!} ||varphi_1 - varphi_2 ||, forall t in I_a \
|| Gamma^{m} varphi_1 - Gamma^{m} varphi_2 || &leq dfrac{L^{m} a^{m}}{m!} ||varphi_1 - varphi_2 ||
end{align}
$endgroup$
add a comment |
$begingroup$
First, let's prove this:
$$
|| Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || leq dfrac{L^{m} |t-t_0|^{m}}{m!} ||varphi_1 - varphi_2 ||, forall t in I_a
$$
With $m = 1$, is trivial. If the induction hypothesis is:
$$|| Gamma^{m-1} varphi_1(t) - Gamma^{m-1} varphi_2(t) || leq dfrac{L^{m-1} |t-t_0|^{m-1}}{(m-1)!} ||varphi_1 - varphi_2 ||, forall t in I_a
$$
Then $forall t in I_a$:
begin{align}
|| Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || = left|left| GammaGamma^{m-1} varphi_1(t) - Gamma Gamma^{m-1} varphi_2(t) right|right| &= left|left| int_{t_0}^{t} fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) ds right|right| \
&leq left| int_{t_0}^{t} left|left| fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) right|right| ds right|
end{align}
But $f$ is Lipschitz in $I_a times B_b$ in the second variable with $L$ as Lipschitz constant. Then:
begin{align}
left| int_{t_0}^{t} left|left| fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) right|right| ds right|
&leq
L left| int_{t_0}^{t} left|left| Gamma^{m-1} varphi_1(s) - Gamma^{m-1} varphi_2(s) right|right| ds right| \
end{align}
And by induction hypothesis:
begin{align}
left| int_{t_0}^{t} left|left| fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) right|right| ds right|
&leq
L left| int_{t_0}^{t} dfrac{L^{m-1} |s-t_0|^{m-1}}{(m-1)!} ||varphi_1 - varphi_2 || ds right| \
&leq
dfrac{L^m}{(m-1)!} left| int_{t_0}^{t} |s-t_0|^{m-1} ||varphi_1 - varphi_2 || ds right| \
&leq
frac{L^m|t-t_0|^m}{m!} ||varphi_1 - varphi_2 || \
end{align}
Then the previous proposition is right, now:
begin{align}
|| Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || leq dfrac{L^{m} |t-t_0|^{m}}{m!} ||varphi_1 - varphi_2 || leq dfrac{L^{m} a^{m}}{m!} ||varphi_1 - varphi_2 ||, forall t in I_a
end{align}
Then:
begin{align}
textrm{sup} left{ || Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || : t in I_a right} &leq dfrac{L^{m} a^{m}}{m!} ||varphi_1 - varphi_2 ||, forall t in I_a \
|| Gamma^{m} varphi_1 - Gamma^{m} varphi_2 || &leq dfrac{L^{m} a^{m}}{m!} ||varphi_1 - varphi_2 ||
end{align}
$endgroup$
First, let's prove this:
$$
|| Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || leq dfrac{L^{m} |t-t_0|^{m}}{m!} ||varphi_1 - varphi_2 ||, forall t in I_a
$$
With $m = 1$, is trivial. If the induction hypothesis is:
$$|| Gamma^{m-1} varphi_1(t) - Gamma^{m-1} varphi_2(t) || leq dfrac{L^{m-1} |t-t_0|^{m-1}}{(m-1)!} ||varphi_1 - varphi_2 ||, forall t in I_a
$$
Then $forall t in I_a$:
begin{align}
|| Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || = left|left| GammaGamma^{m-1} varphi_1(t) - Gamma Gamma^{m-1} varphi_2(t) right|right| &= left|left| int_{t_0}^{t} fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) ds right|right| \
&leq left| int_{t_0}^{t} left|left| fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) right|right| ds right|
end{align}
But $f$ is Lipschitz in $I_a times B_b$ in the second variable with $L$ as Lipschitz constant. Then:
begin{align}
left| int_{t_0}^{t} left|left| fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) right|right| ds right|
&leq
L left| int_{t_0}^{t} left|left| Gamma^{m-1} varphi_1(s) - Gamma^{m-1} varphi_2(s) right|right| ds right| \
end{align}
And by induction hypothesis:
begin{align}
left| int_{t_0}^{t} left|left| fleft(s,Gamma^{m-1} varphi_1(s) right) - fleft(s,Gamma^{m-1} varphi_2(s) right) right|right| ds right|
&leq
L left| int_{t_0}^{t} dfrac{L^{m-1} |s-t_0|^{m-1}}{(m-1)!} ||varphi_1 - varphi_2 || ds right| \
&leq
dfrac{L^m}{(m-1)!} left| int_{t_0}^{t} |s-t_0|^{m-1} ||varphi_1 - varphi_2 || ds right| \
&leq
frac{L^m|t-t_0|^m}{m!} ||varphi_1 - varphi_2 || \
end{align}
Then the previous proposition is right, now:
begin{align}
|| Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || leq dfrac{L^{m} |t-t_0|^{m}}{m!} ||varphi_1 - varphi_2 || leq dfrac{L^{m} a^{m}}{m!} ||varphi_1 - varphi_2 ||, forall t in I_a
end{align}
Then:
begin{align}
textrm{sup} left{ || Gamma^{m} varphi_1(t) - Gamma^{m} varphi_2(t) || : t in I_a right} &leq dfrac{L^{m} a^{m}}{m!} ||varphi_1 - varphi_2 ||, forall t in I_a \
|| Gamma^{m} varphi_1 - Gamma^{m} varphi_2 || &leq dfrac{L^{m} a^{m}}{m!} ||varphi_1 - varphi_2 ||
end{align}
answered Dec 30 '18 at 7:31
El boritoEl borito
666216
666216
add a comment |
add a comment |
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$begingroup$
There is also a proof given in the section you cite. Have you read it? Where do you disagree?
$endgroup$
– martini
Jul 15 '14 at 10:03
$begingroup$
m! term should not appear in the denominator.
$endgroup$
– pitonist
Jul 15 '14 at 10:07
1
$begingroup$
I think one should replace the claim $$ |Gamma^m phi_1 - Gamma^m phi_2 | le frac{ L^m alpha^m}{m!}|phi_1-phi_2| $$ by $$ |Gamma^m phi_1(s) - Gamma^m phi_2(s) | le frac{ L^m}{m!}(s-t_0)^m|phi_1-phi_2| quad forall sin(t_0,t_0+a). $$ Very sloppy proof there.
$endgroup$
– daw
Jul 15 '14 at 10:20