Proving that a solution to a differential equation is monotonic
$begingroup$
The answer that ws given on a previous question of mine, stated that the solution to this DE:
$$x(t)cdot r+x'(t)cdot l+acdotlnleft(1+frac{x(t)}{b}right)=0spaceLongleftrightarrowspace x(t)=dotstag1$$
Must be monotonic.
Is there a way to proof that that is the case, that the function $x(t)$ is monotonic?!
Background:
I've to find (the average of a function over a particular interval, where $t_1>0$, $t_2>0$ and $t_2>t_1$):
$$frac{1}{t_2-t_1}int_{t_1}^{t_2}x(t)dttag2$$
Where $x(t)$ in equation $(2)$ is the solution to the DE in equation $(1)$.
ordinary-differential-equations definite-integrals logarithms mathematical-physics average
$endgroup$
add a comment |
$begingroup$
The answer that ws given on a previous question of mine, stated that the solution to this DE:
$$x(t)cdot r+x'(t)cdot l+acdotlnleft(1+frac{x(t)}{b}right)=0spaceLongleftrightarrowspace x(t)=dotstag1$$
Must be monotonic.
Is there a way to proof that that is the case, that the function $x(t)$ is monotonic?!
Background:
I've to find (the average of a function over a particular interval, where $t_1>0$, $t_2>0$ and $t_2>t_1$):
$$frac{1}{t_2-t_1}int_{t_1}^{t_2}x(t)dttag2$$
Where $x(t)$ in equation $(2)$ is the solution to the DE in equation $(1)$.
ordinary-differential-equations definite-integrals logarithms mathematical-physics average
$endgroup$
add a comment |
$begingroup$
The answer that ws given on a previous question of mine, stated that the solution to this DE:
$$x(t)cdot r+x'(t)cdot l+acdotlnleft(1+frac{x(t)}{b}right)=0spaceLongleftrightarrowspace x(t)=dotstag1$$
Must be monotonic.
Is there a way to proof that that is the case, that the function $x(t)$ is monotonic?!
Background:
I've to find (the average of a function over a particular interval, where $t_1>0$, $t_2>0$ and $t_2>t_1$):
$$frac{1}{t_2-t_1}int_{t_1}^{t_2}x(t)dttag2$$
Where $x(t)$ in equation $(2)$ is the solution to the DE in equation $(1)$.
ordinary-differential-equations definite-integrals logarithms mathematical-physics average
$endgroup$
The answer that ws given on a previous question of mine, stated that the solution to this DE:
$$x(t)cdot r+x'(t)cdot l+acdotlnleft(1+frac{x(t)}{b}right)=0spaceLongleftrightarrowspace x(t)=dotstag1$$
Must be monotonic.
Is there a way to proof that that is the case, that the function $x(t)$ is monotonic?!
Background:
I've to find (the average of a function over a particular interval, where $t_1>0$, $t_2>0$ and $t_2>t_1$):
$$frac{1}{t_2-t_1}int_{t_1}^{t_2}x(t)dttag2$$
Where $x(t)$ in equation $(2)$ is the solution to the DE in equation $(1)$.
ordinary-differential-equations definite-integrals logarithms mathematical-physics average
ordinary-differential-equations definite-integrals logarithms mathematical-physics average
edited Dec 31 '18 at 17:29
David G. Stork
11k41432
11k41432
asked Dec 31 '18 at 17:23
KlopjasKlopjas
714
714
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Considering the DE
$$
l x'+r x+alnleft(1+frac xaright)=0
$$
taking it's derivative (assuming $x$ is twice diferentiable)
$$
frac{a x'}{b left(frac{x}{b}+1right)}+l x''+r x' = 0
$$
after substituting $x'$ from the first equation we get
$$
l x''= frac{(a+b r+r x) left(a ln left(frac{b+x}{b}right)+r xright)}{l (b+x)}
$$
and now solving for $x$ the condition $lx'' = 0$ we obtain
$$
x^* = {frac ar Wleft(frac{b e^{frac{b r}{a}} r}{a}right)-b,-left(b+frac arright)}
$$
also regarding the first equation we have
$$
l x'=-left(r x+alnleft(1+frac xaright)right) = 0Rightarrow x^* = frac ar Wleft(frac{b e^{frac{b r}{a}} r}{a}right)-b
$$
so the only stationary point has null $x''$ so it is an inflexion point indicating that $x$ is monotonic..
$endgroup$
$begingroup$
You assumed it's double differentiable.
$endgroup$
– Lucas Henrique
Dec 31 '18 at 18:37
$begingroup$
@LucasHenrique Yes. I assumed that. (Included in the last edit).
$endgroup$
– Cesareo
Dec 31 '18 at 18:53
$begingroup$
Why did you take $lx'' = 0$?
$endgroup$
– Lucas Henrique
Dec 31 '18 at 20:51
$begingroup$
@LucasHenrique I think that now the conclusion is the correct one.
$endgroup$
– Cesareo
Dec 31 '18 at 21:53
$begingroup$
@Cesareo So $x(t)$ is monotonous? As I conclude from your answer?!
$endgroup$
– Klopjas
Dec 31 '18 at 21:54
|
show 1 more comment
$begingroup$
A differentiable function is not monotonic if there are two different points whose derivatives have different signs. Since $x' = -frac{rx + aln ( 1 + x/b)}{l}$ is a composition of differentiable functions, it's differentiable and thus continuous; if not monotonic, $x'$ it must have a root; and at least one root is not an inflection point. Simbolically, $$begin{cases} x'(t) = 0 \ x''(t) neq 0end{cases}$$ for some $t$. But $x'' = -x'left(r + frac{a}{b + x}right)$ so the second derivative is zero on all the roots of the first derivative. So every root is an inflection point and thus, in fact, $x$ is monotonic.
$endgroup$
$begingroup$
OP: your function is monotonic.
$endgroup$
– Lucas Henrique
Dec 31 '18 at 23:54
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3057878%2fproving-that-a-solution-to-a-differential-equation-is-monotonic%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Considering the DE
$$
l x'+r x+alnleft(1+frac xaright)=0
$$
taking it's derivative (assuming $x$ is twice diferentiable)
$$
frac{a x'}{b left(frac{x}{b}+1right)}+l x''+r x' = 0
$$
after substituting $x'$ from the first equation we get
$$
l x''= frac{(a+b r+r x) left(a ln left(frac{b+x}{b}right)+r xright)}{l (b+x)}
$$
and now solving for $x$ the condition $lx'' = 0$ we obtain
$$
x^* = {frac ar Wleft(frac{b e^{frac{b r}{a}} r}{a}right)-b,-left(b+frac arright)}
$$
also regarding the first equation we have
$$
l x'=-left(r x+alnleft(1+frac xaright)right) = 0Rightarrow x^* = frac ar Wleft(frac{b e^{frac{b r}{a}} r}{a}right)-b
$$
so the only stationary point has null $x''$ so it is an inflexion point indicating that $x$ is monotonic..
$endgroup$
$begingroup$
You assumed it's double differentiable.
$endgroup$
– Lucas Henrique
Dec 31 '18 at 18:37
$begingroup$
@LucasHenrique Yes. I assumed that. (Included in the last edit).
$endgroup$
– Cesareo
Dec 31 '18 at 18:53
$begingroup$
Why did you take $lx'' = 0$?
$endgroup$
– Lucas Henrique
Dec 31 '18 at 20:51
$begingroup$
@LucasHenrique I think that now the conclusion is the correct one.
$endgroup$
– Cesareo
Dec 31 '18 at 21:53
$begingroup$
@Cesareo So $x(t)$ is monotonous? As I conclude from your answer?!
$endgroup$
– Klopjas
Dec 31 '18 at 21:54
|
show 1 more comment
$begingroup$
Considering the DE
$$
l x'+r x+alnleft(1+frac xaright)=0
$$
taking it's derivative (assuming $x$ is twice diferentiable)
$$
frac{a x'}{b left(frac{x}{b}+1right)}+l x''+r x' = 0
$$
after substituting $x'$ from the first equation we get
$$
l x''= frac{(a+b r+r x) left(a ln left(frac{b+x}{b}right)+r xright)}{l (b+x)}
$$
and now solving for $x$ the condition $lx'' = 0$ we obtain
$$
x^* = {frac ar Wleft(frac{b e^{frac{b r}{a}} r}{a}right)-b,-left(b+frac arright)}
$$
also regarding the first equation we have
$$
l x'=-left(r x+alnleft(1+frac xaright)right) = 0Rightarrow x^* = frac ar Wleft(frac{b e^{frac{b r}{a}} r}{a}right)-b
$$
so the only stationary point has null $x''$ so it is an inflexion point indicating that $x$ is monotonic..
$endgroup$
$begingroup$
You assumed it's double differentiable.
$endgroup$
– Lucas Henrique
Dec 31 '18 at 18:37
$begingroup$
@LucasHenrique Yes. I assumed that. (Included in the last edit).
$endgroup$
– Cesareo
Dec 31 '18 at 18:53
$begingroup$
Why did you take $lx'' = 0$?
$endgroup$
– Lucas Henrique
Dec 31 '18 at 20:51
$begingroup$
@LucasHenrique I think that now the conclusion is the correct one.
$endgroup$
– Cesareo
Dec 31 '18 at 21:53
$begingroup$
@Cesareo So $x(t)$ is monotonous? As I conclude from your answer?!
$endgroup$
– Klopjas
Dec 31 '18 at 21:54
|
show 1 more comment
$begingroup$
Considering the DE
$$
l x'+r x+alnleft(1+frac xaright)=0
$$
taking it's derivative (assuming $x$ is twice diferentiable)
$$
frac{a x'}{b left(frac{x}{b}+1right)}+l x''+r x' = 0
$$
after substituting $x'$ from the first equation we get
$$
l x''= frac{(a+b r+r x) left(a ln left(frac{b+x}{b}right)+r xright)}{l (b+x)}
$$
and now solving for $x$ the condition $lx'' = 0$ we obtain
$$
x^* = {frac ar Wleft(frac{b e^{frac{b r}{a}} r}{a}right)-b,-left(b+frac arright)}
$$
also regarding the first equation we have
$$
l x'=-left(r x+alnleft(1+frac xaright)right) = 0Rightarrow x^* = frac ar Wleft(frac{b e^{frac{b r}{a}} r}{a}right)-b
$$
so the only stationary point has null $x''$ so it is an inflexion point indicating that $x$ is monotonic..
$endgroup$
Considering the DE
$$
l x'+r x+alnleft(1+frac xaright)=0
$$
taking it's derivative (assuming $x$ is twice diferentiable)
$$
frac{a x'}{b left(frac{x}{b}+1right)}+l x''+r x' = 0
$$
after substituting $x'$ from the first equation we get
$$
l x''= frac{(a+b r+r x) left(a ln left(frac{b+x}{b}right)+r xright)}{l (b+x)}
$$
and now solving for $x$ the condition $lx'' = 0$ we obtain
$$
x^* = {frac ar Wleft(frac{b e^{frac{b r}{a}} r}{a}right)-b,-left(b+frac arright)}
$$
also regarding the first equation we have
$$
l x'=-left(r x+alnleft(1+frac xaright)right) = 0Rightarrow x^* = frac ar Wleft(frac{b e^{frac{b r}{a}} r}{a}right)-b
$$
so the only stationary point has null $x''$ so it is an inflexion point indicating that $x$ is monotonic..
edited Dec 31 '18 at 21:48
answered Dec 31 '18 at 18:19
CesareoCesareo
9,1063517
9,1063517
$begingroup$
You assumed it's double differentiable.
$endgroup$
– Lucas Henrique
Dec 31 '18 at 18:37
$begingroup$
@LucasHenrique Yes. I assumed that. (Included in the last edit).
$endgroup$
– Cesareo
Dec 31 '18 at 18:53
$begingroup$
Why did you take $lx'' = 0$?
$endgroup$
– Lucas Henrique
Dec 31 '18 at 20:51
$begingroup$
@LucasHenrique I think that now the conclusion is the correct one.
$endgroup$
– Cesareo
Dec 31 '18 at 21:53
$begingroup$
@Cesareo So $x(t)$ is monotonous? As I conclude from your answer?!
$endgroup$
– Klopjas
Dec 31 '18 at 21:54
|
show 1 more comment
$begingroup$
You assumed it's double differentiable.
$endgroup$
– Lucas Henrique
Dec 31 '18 at 18:37
$begingroup$
@LucasHenrique Yes. I assumed that. (Included in the last edit).
$endgroup$
– Cesareo
Dec 31 '18 at 18:53
$begingroup$
Why did you take $lx'' = 0$?
$endgroup$
– Lucas Henrique
Dec 31 '18 at 20:51
$begingroup$
@LucasHenrique I think that now the conclusion is the correct one.
$endgroup$
– Cesareo
Dec 31 '18 at 21:53
$begingroup$
@Cesareo So $x(t)$ is monotonous? As I conclude from your answer?!
$endgroup$
– Klopjas
Dec 31 '18 at 21:54
$begingroup$
You assumed it's double differentiable.
$endgroup$
– Lucas Henrique
Dec 31 '18 at 18:37
$begingroup$
You assumed it's double differentiable.
$endgroup$
– Lucas Henrique
Dec 31 '18 at 18:37
$begingroup$
@LucasHenrique Yes. I assumed that. (Included in the last edit).
$endgroup$
– Cesareo
Dec 31 '18 at 18:53
$begingroup$
@LucasHenrique Yes. I assumed that. (Included in the last edit).
$endgroup$
– Cesareo
Dec 31 '18 at 18:53
$begingroup$
Why did you take $lx'' = 0$?
$endgroup$
– Lucas Henrique
Dec 31 '18 at 20:51
$begingroup$
Why did you take $lx'' = 0$?
$endgroup$
– Lucas Henrique
Dec 31 '18 at 20:51
$begingroup$
@LucasHenrique I think that now the conclusion is the correct one.
$endgroup$
– Cesareo
Dec 31 '18 at 21:53
$begingroup$
@LucasHenrique I think that now the conclusion is the correct one.
$endgroup$
– Cesareo
Dec 31 '18 at 21:53
$begingroup$
@Cesareo So $x(t)$ is monotonous? As I conclude from your answer?!
$endgroup$
– Klopjas
Dec 31 '18 at 21:54
$begingroup$
@Cesareo So $x(t)$ is monotonous? As I conclude from your answer?!
$endgroup$
– Klopjas
Dec 31 '18 at 21:54
|
show 1 more comment
$begingroup$
A differentiable function is not monotonic if there are two different points whose derivatives have different signs. Since $x' = -frac{rx + aln ( 1 + x/b)}{l}$ is a composition of differentiable functions, it's differentiable and thus continuous; if not monotonic, $x'$ it must have a root; and at least one root is not an inflection point. Simbolically, $$begin{cases} x'(t) = 0 \ x''(t) neq 0end{cases}$$ for some $t$. But $x'' = -x'left(r + frac{a}{b + x}right)$ so the second derivative is zero on all the roots of the first derivative. So every root is an inflection point and thus, in fact, $x$ is monotonic.
$endgroup$
$begingroup$
OP: your function is monotonic.
$endgroup$
– Lucas Henrique
Dec 31 '18 at 23:54
add a comment |
$begingroup$
A differentiable function is not monotonic if there are two different points whose derivatives have different signs. Since $x' = -frac{rx + aln ( 1 + x/b)}{l}$ is a composition of differentiable functions, it's differentiable and thus continuous; if not monotonic, $x'$ it must have a root; and at least one root is not an inflection point. Simbolically, $$begin{cases} x'(t) = 0 \ x''(t) neq 0end{cases}$$ for some $t$. But $x'' = -x'left(r + frac{a}{b + x}right)$ so the second derivative is zero on all the roots of the first derivative. So every root is an inflection point and thus, in fact, $x$ is monotonic.
$endgroup$
$begingroup$
OP: your function is monotonic.
$endgroup$
– Lucas Henrique
Dec 31 '18 at 23:54
add a comment |
$begingroup$
A differentiable function is not monotonic if there are two different points whose derivatives have different signs. Since $x' = -frac{rx + aln ( 1 + x/b)}{l}$ is a composition of differentiable functions, it's differentiable and thus continuous; if not monotonic, $x'$ it must have a root; and at least one root is not an inflection point. Simbolically, $$begin{cases} x'(t) = 0 \ x''(t) neq 0end{cases}$$ for some $t$. But $x'' = -x'left(r + frac{a}{b + x}right)$ so the second derivative is zero on all the roots of the first derivative. So every root is an inflection point and thus, in fact, $x$ is monotonic.
$endgroup$
A differentiable function is not monotonic if there are two different points whose derivatives have different signs. Since $x' = -frac{rx + aln ( 1 + x/b)}{l}$ is a composition of differentiable functions, it's differentiable and thus continuous; if not monotonic, $x'$ it must have a root; and at least one root is not an inflection point. Simbolically, $$begin{cases} x'(t) = 0 \ x''(t) neq 0end{cases}$$ for some $t$. But $x'' = -x'left(r + frac{a}{b + x}right)$ so the second derivative is zero on all the roots of the first derivative. So every root is an inflection point and thus, in fact, $x$ is monotonic.
edited Dec 31 '18 at 23:50
answered Dec 31 '18 at 22:16
Lucas HenriqueLucas Henrique
1,026414
1,026414
$begingroup$
OP: your function is monotonic.
$endgroup$
– Lucas Henrique
Dec 31 '18 at 23:54
add a comment |
$begingroup$
OP: your function is monotonic.
$endgroup$
– Lucas Henrique
Dec 31 '18 at 23:54
$begingroup$
OP: your function is monotonic.
$endgroup$
– Lucas Henrique
Dec 31 '18 at 23:54
$begingroup$
OP: your function is monotonic.
$endgroup$
– Lucas Henrique
Dec 31 '18 at 23:54
add a comment |
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3057878%2fproving-that-a-solution-to-a-differential-equation-is-monotonic%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown