Fundamental matrix of Hill's equation
$begingroup$
The Hill's equation $(H)$ is defined as $y'' + p(t)y=0$ where $p(t+T)=p(t) forall t$. Let $x_1=y$ and $x_2=y'$, and let $x=begin{bmatrix}{x_1}\{x_2}end{bmatrix}$, so $x'=begin{bmatrix}{0}&{1}\{-p(t)}&{0}end{bmatrix}x$.
Now we consider the solutions of $(H)$ with the following initial conditions:
$varphi_1(0) = 1$, $varphi_2(0) = 0$, $varphi_1'(0) = 0$ and $varphi_2'(0) = 1$.
My Floquet's Theory notes says that $phi(t) = begin{bmatrix}{varphi_1}&{varphi_2}\{varphi_1'}&{varphi_2'}end{bmatrix}$ is a fundamental matrix of $(H)$, but I don't know why. Could anybody explain that? Thanks
differential
$endgroup$
add a comment |
$begingroup$
The Hill's equation $(H)$ is defined as $y'' + p(t)y=0$ where $p(t+T)=p(t) forall t$. Let $x_1=y$ and $x_2=y'$, and let $x=begin{bmatrix}{x_1}\{x_2}end{bmatrix}$, so $x'=begin{bmatrix}{0}&{1}\{-p(t)}&{0}end{bmatrix}x$.
Now we consider the solutions of $(H)$ with the following initial conditions:
$varphi_1(0) = 1$, $varphi_2(0) = 0$, $varphi_1'(0) = 0$ and $varphi_2'(0) = 1$.
My Floquet's Theory notes says that $phi(t) = begin{bmatrix}{varphi_1}&{varphi_2}\{varphi_1'}&{varphi_2'}end{bmatrix}$ is a fundamental matrix of $(H)$, but I don't know why. Could anybody explain that? Thanks
differential
$endgroup$
add a comment |
$begingroup$
The Hill's equation $(H)$ is defined as $y'' + p(t)y=0$ where $p(t+T)=p(t) forall t$. Let $x_1=y$ and $x_2=y'$, and let $x=begin{bmatrix}{x_1}\{x_2}end{bmatrix}$, so $x'=begin{bmatrix}{0}&{1}\{-p(t)}&{0}end{bmatrix}x$.
Now we consider the solutions of $(H)$ with the following initial conditions:
$varphi_1(0) = 1$, $varphi_2(0) = 0$, $varphi_1'(0) = 0$ and $varphi_2'(0) = 1$.
My Floquet's Theory notes says that $phi(t) = begin{bmatrix}{varphi_1}&{varphi_2}\{varphi_1'}&{varphi_2'}end{bmatrix}$ is a fundamental matrix of $(H)$, but I don't know why. Could anybody explain that? Thanks
differential
$endgroup$
The Hill's equation $(H)$ is defined as $y'' + p(t)y=0$ where $p(t+T)=p(t) forall t$. Let $x_1=y$ and $x_2=y'$, and let $x=begin{bmatrix}{x_1}\{x_2}end{bmatrix}$, so $x'=begin{bmatrix}{0}&{1}\{-p(t)}&{0}end{bmatrix}x$.
Now we consider the solutions of $(H)$ with the following initial conditions:
$varphi_1(0) = 1$, $varphi_2(0) = 0$, $varphi_1'(0) = 0$ and $varphi_2'(0) = 1$.
My Floquet's Theory notes says that $phi(t) = begin{bmatrix}{varphi_1}&{varphi_2}\{varphi_1'}&{varphi_2'}end{bmatrix}$ is a fundamental matrix of $(H)$, but I don't know why. Could anybody explain that? Thanks
differential
differential
edited Jan 2 at 22:11
vicase98
asked Dec 31 '18 at 13:09
vicase98vicase98
214
214
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
The condition that $p(t)$ is periodic is not necessary. You also have an error in the definition of $varphi_1, varphi_2$.
Consider a linear system of ODEs, $x'(t)=A(t)x(t)$, where $x(t)$ is an $mathbb{R}^n$-valued unknown function and $A(t)$ is a given $ntimes n$ matrix-valued function. This system defines for each $tinmathbb{R}$ a linear transformation $F(t):mathbb{R}^ntomathbb{R}^n$, sending $vinmathbb{R}^n$ to the value at $t$ of the solution to the system whose value at $t=0$ is $v$. The matrix of $F(t)$ with respect to the standard basis of $mathbb{R}^n$, as a function of $t$, is called the fundamental matrix of solutions of the system. Its columns form a basis of the space of solutions of the system. The $j$-th column is the solution whose value at $t=0$ is the $j$-th standard vector (1 at the $j$-th place, the rest 0). If you apply this to the Hill's equation you get that, in your notation, $varphi_1(0)=1,varphi'_1(0)=0, varphi_2(0)=0,varphi'_2(0)=1$.
$endgroup$
$begingroup$
It's true, the initial conditions were wrong, sorry for the mistake and than you for the explanation. Anyway, would you mind explainning why that $phi(t)$ satisfies the equation?I will really appreciate it
$endgroup$
– vicase98
Jan 2 at 20:58
$begingroup$
Just chasing definitions. $phi(t)$, by definition, is the matrix of $F(t)$ with respect to the standard basis of $mathbb{R}^n$. That is, the $j$-th column of $phi(t)$ is the image of the $j$-th standard basis vector under $F(t)$. By definition of $F(t)$, it is the solution to $x'=Ax$ such that $x(0)$ is the standard $j$-th vector. This is the same as $phi'=Aphi$, with $phi(0)=I_n$ (the $n$ by $n$ identity matrix).
$endgroup$
– Gil Bor
Jan 2 at 21:17
$begingroup$
I mean the Hill's equation, not $x'=Ax$
$endgroup$
– vicase98
Jan 3 at 8:23
$begingroup$
Can you define ''the fundamental matrix of of (H)''? As far as I know, the fundamental matrix of a 2nd order ODE is defined as the fundamental matrix of the associated 1st order linear system. In other words, the fundamental matrix $phi(t)$ of $y''+py=0$ is not supposed to satisfy $phi''+pphi=0$, but rather $phi'=Aphi$, with $A$ the 2 by 2 matrix in your question.
$endgroup$
– Gil Bor
Jan 5 at 7:54
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%2f3057696%2ffundamental-matrix-of-hills-equation%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
The condition that $p(t)$ is periodic is not necessary. You also have an error in the definition of $varphi_1, varphi_2$.
Consider a linear system of ODEs, $x'(t)=A(t)x(t)$, where $x(t)$ is an $mathbb{R}^n$-valued unknown function and $A(t)$ is a given $ntimes n$ matrix-valued function. This system defines for each $tinmathbb{R}$ a linear transformation $F(t):mathbb{R}^ntomathbb{R}^n$, sending $vinmathbb{R}^n$ to the value at $t$ of the solution to the system whose value at $t=0$ is $v$. The matrix of $F(t)$ with respect to the standard basis of $mathbb{R}^n$, as a function of $t$, is called the fundamental matrix of solutions of the system. Its columns form a basis of the space of solutions of the system. The $j$-th column is the solution whose value at $t=0$ is the $j$-th standard vector (1 at the $j$-th place, the rest 0). If you apply this to the Hill's equation you get that, in your notation, $varphi_1(0)=1,varphi'_1(0)=0, varphi_2(0)=0,varphi'_2(0)=1$.
$endgroup$
$begingroup$
It's true, the initial conditions were wrong, sorry for the mistake and than you for the explanation. Anyway, would you mind explainning why that $phi(t)$ satisfies the equation?I will really appreciate it
$endgroup$
– vicase98
Jan 2 at 20:58
$begingroup$
Just chasing definitions. $phi(t)$, by definition, is the matrix of $F(t)$ with respect to the standard basis of $mathbb{R}^n$. That is, the $j$-th column of $phi(t)$ is the image of the $j$-th standard basis vector under $F(t)$. By definition of $F(t)$, it is the solution to $x'=Ax$ such that $x(0)$ is the standard $j$-th vector. This is the same as $phi'=Aphi$, with $phi(0)=I_n$ (the $n$ by $n$ identity matrix).
$endgroup$
– Gil Bor
Jan 2 at 21:17
$begingroup$
I mean the Hill's equation, not $x'=Ax$
$endgroup$
– vicase98
Jan 3 at 8:23
$begingroup$
Can you define ''the fundamental matrix of of (H)''? As far as I know, the fundamental matrix of a 2nd order ODE is defined as the fundamental matrix of the associated 1st order linear system. In other words, the fundamental matrix $phi(t)$ of $y''+py=0$ is not supposed to satisfy $phi''+pphi=0$, but rather $phi'=Aphi$, with $A$ the 2 by 2 matrix in your question.
$endgroup$
– Gil Bor
Jan 5 at 7:54
add a comment |
$begingroup$
The condition that $p(t)$ is periodic is not necessary. You also have an error in the definition of $varphi_1, varphi_2$.
Consider a linear system of ODEs, $x'(t)=A(t)x(t)$, where $x(t)$ is an $mathbb{R}^n$-valued unknown function and $A(t)$ is a given $ntimes n$ matrix-valued function. This system defines for each $tinmathbb{R}$ a linear transformation $F(t):mathbb{R}^ntomathbb{R}^n$, sending $vinmathbb{R}^n$ to the value at $t$ of the solution to the system whose value at $t=0$ is $v$. The matrix of $F(t)$ with respect to the standard basis of $mathbb{R}^n$, as a function of $t$, is called the fundamental matrix of solutions of the system. Its columns form a basis of the space of solutions of the system. The $j$-th column is the solution whose value at $t=0$ is the $j$-th standard vector (1 at the $j$-th place, the rest 0). If you apply this to the Hill's equation you get that, in your notation, $varphi_1(0)=1,varphi'_1(0)=0, varphi_2(0)=0,varphi'_2(0)=1$.
$endgroup$
$begingroup$
It's true, the initial conditions were wrong, sorry for the mistake and than you for the explanation. Anyway, would you mind explainning why that $phi(t)$ satisfies the equation?I will really appreciate it
$endgroup$
– vicase98
Jan 2 at 20:58
$begingroup$
Just chasing definitions. $phi(t)$, by definition, is the matrix of $F(t)$ with respect to the standard basis of $mathbb{R}^n$. That is, the $j$-th column of $phi(t)$ is the image of the $j$-th standard basis vector under $F(t)$. By definition of $F(t)$, it is the solution to $x'=Ax$ such that $x(0)$ is the standard $j$-th vector. This is the same as $phi'=Aphi$, with $phi(0)=I_n$ (the $n$ by $n$ identity matrix).
$endgroup$
– Gil Bor
Jan 2 at 21:17
$begingroup$
I mean the Hill's equation, not $x'=Ax$
$endgroup$
– vicase98
Jan 3 at 8:23
$begingroup$
Can you define ''the fundamental matrix of of (H)''? As far as I know, the fundamental matrix of a 2nd order ODE is defined as the fundamental matrix of the associated 1st order linear system. In other words, the fundamental matrix $phi(t)$ of $y''+py=0$ is not supposed to satisfy $phi''+pphi=0$, but rather $phi'=Aphi$, with $A$ the 2 by 2 matrix in your question.
$endgroup$
– Gil Bor
Jan 5 at 7:54
add a comment |
$begingroup$
The condition that $p(t)$ is periodic is not necessary. You also have an error in the definition of $varphi_1, varphi_2$.
Consider a linear system of ODEs, $x'(t)=A(t)x(t)$, where $x(t)$ is an $mathbb{R}^n$-valued unknown function and $A(t)$ is a given $ntimes n$ matrix-valued function. This system defines for each $tinmathbb{R}$ a linear transformation $F(t):mathbb{R}^ntomathbb{R}^n$, sending $vinmathbb{R}^n$ to the value at $t$ of the solution to the system whose value at $t=0$ is $v$. The matrix of $F(t)$ with respect to the standard basis of $mathbb{R}^n$, as a function of $t$, is called the fundamental matrix of solutions of the system. Its columns form a basis of the space of solutions of the system. The $j$-th column is the solution whose value at $t=0$ is the $j$-th standard vector (1 at the $j$-th place, the rest 0). If you apply this to the Hill's equation you get that, in your notation, $varphi_1(0)=1,varphi'_1(0)=0, varphi_2(0)=0,varphi'_2(0)=1$.
$endgroup$
The condition that $p(t)$ is periodic is not necessary. You also have an error in the definition of $varphi_1, varphi_2$.
Consider a linear system of ODEs, $x'(t)=A(t)x(t)$, where $x(t)$ is an $mathbb{R}^n$-valued unknown function and $A(t)$ is a given $ntimes n$ matrix-valued function. This system defines for each $tinmathbb{R}$ a linear transformation $F(t):mathbb{R}^ntomathbb{R}^n$, sending $vinmathbb{R}^n$ to the value at $t$ of the solution to the system whose value at $t=0$ is $v$. The matrix of $F(t)$ with respect to the standard basis of $mathbb{R}^n$, as a function of $t$, is called the fundamental matrix of solutions of the system. Its columns form a basis of the space of solutions of the system. The $j$-th column is the solution whose value at $t=0$ is the $j$-th standard vector (1 at the $j$-th place, the rest 0). If you apply this to the Hill's equation you get that, in your notation, $varphi_1(0)=1,varphi'_1(0)=0, varphi_2(0)=0,varphi'_2(0)=1$.
answered Jan 2 at 19:48
Gil BorGil Bor
2,985812
2,985812
$begingroup$
It's true, the initial conditions were wrong, sorry for the mistake and than you for the explanation. Anyway, would you mind explainning why that $phi(t)$ satisfies the equation?I will really appreciate it
$endgroup$
– vicase98
Jan 2 at 20:58
$begingroup$
Just chasing definitions. $phi(t)$, by definition, is the matrix of $F(t)$ with respect to the standard basis of $mathbb{R}^n$. That is, the $j$-th column of $phi(t)$ is the image of the $j$-th standard basis vector under $F(t)$. By definition of $F(t)$, it is the solution to $x'=Ax$ such that $x(0)$ is the standard $j$-th vector. This is the same as $phi'=Aphi$, with $phi(0)=I_n$ (the $n$ by $n$ identity matrix).
$endgroup$
– Gil Bor
Jan 2 at 21:17
$begingroup$
I mean the Hill's equation, not $x'=Ax$
$endgroup$
– vicase98
Jan 3 at 8:23
$begingroup$
Can you define ''the fundamental matrix of of (H)''? As far as I know, the fundamental matrix of a 2nd order ODE is defined as the fundamental matrix of the associated 1st order linear system. In other words, the fundamental matrix $phi(t)$ of $y''+py=0$ is not supposed to satisfy $phi''+pphi=0$, but rather $phi'=Aphi$, with $A$ the 2 by 2 matrix in your question.
$endgroup$
– Gil Bor
Jan 5 at 7:54
add a comment |
$begingroup$
It's true, the initial conditions were wrong, sorry for the mistake and than you for the explanation. Anyway, would you mind explainning why that $phi(t)$ satisfies the equation?I will really appreciate it
$endgroup$
– vicase98
Jan 2 at 20:58
$begingroup$
Just chasing definitions. $phi(t)$, by definition, is the matrix of $F(t)$ with respect to the standard basis of $mathbb{R}^n$. That is, the $j$-th column of $phi(t)$ is the image of the $j$-th standard basis vector under $F(t)$. By definition of $F(t)$, it is the solution to $x'=Ax$ such that $x(0)$ is the standard $j$-th vector. This is the same as $phi'=Aphi$, with $phi(0)=I_n$ (the $n$ by $n$ identity matrix).
$endgroup$
– Gil Bor
Jan 2 at 21:17
$begingroup$
I mean the Hill's equation, not $x'=Ax$
$endgroup$
– vicase98
Jan 3 at 8:23
$begingroup$
Can you define ''the fundamental matrix of of (H)''? As far as I know, the fundamental matrix of a 2nd order ODE is defined as the fundamental matrix of the associated 1st order linear system. In other words, the fundamental matrix $phi(t)$ of $y''+py=0$ is not supposed to satisfy $phi''+pphi=0$, but rather $phi'=Aphi$, with $A$ the 2 by 2 matrix in your question.
$endgroup$
– Gil Bor
Jan 5 at 7:54
$begingroup$
It's true, the initial conditions were wrong, sorry for the mistake and than you for the explanation. Anyway, would you mind explainning why that $phi(t)$ satisfies the equation?I will really appreciate it
$endgroup$
– vicase98
Jan 2 at 20:58
$begingroup$
It's true, the initial conditions were wrong, sorry for the mistake and than you for the explanation. Anyway, would you mind explainning why that $phi(t)$ satisfies the equation?I will really appreciate it
$endgroup$
– vicase98
Jan 2 at 20:58
$begingroup$
Just chasing definitions. $phi(t)$, by definition, is the matrix of $F(t)$ with respect to the standard basis of $mathbb{R}^n$. That is, the $j$-th column of $phi(t)$ is the image of the $j$-th standard basis vector under $F(t)$. By definition of $F(t)$, it is the solution to $x'=Ax$ such that $x(0)$ is the standard $j$-th vector. This is the same as $phi'=Aphi$, with $phi(0)=I_n$ (the $n$ by $n$ identity matrix).
$endgroup$
– Gil Bor
Jan 2 at 21:17
$begingroup$
Just chasing definitions. $phi(t)$, by definition, is the matrix of $F(t)$ with respect to the standard basis of $mathbb{R}^n$. That is, the $j$-th column of $phi(t)$ is the image of the $j$-th standard basis vector under $F(t)$. By definition of $F(t)$, it is the solution to $x'=Ax$ such that $x(0)$ is the standard $j$-th vector. This is the same as $phi'=Aphi$, with $phi(0)=I_n$ (the $n$ by $n$ identity matrix).
$endgroup$
– Gil Bor
Jan 2 at 21:17
$begingroup$
I mean the Hill's equation, not $x'=Ax$
$endgroup$
– vicase98
Jan 3 at 8:23
$begingroup$
I mean the Hill's equation, not $x'=Ax$
$endgroup$
– vicase98
Jan 3 at 8:23
$begingroup$
Can you define ''the fundamental matrix of of (H)''? As far as I know, the fundamental matrix of a 2nd order ODE is defined as the fundamental matrix of the associated 1st order linear system. In other words, the fundamental matrix $phi(t)$ of $y''+py=0$ is not supposed to satisfy $phi''+pphi=0$, but rather $phi'=Aphi$, with $A$ the 2 by 2 matrix in your question.
$endgroup$
– Gil Bor
Jan 5 at 7:54
$begingroup$
Can you define ''the fundamental matrix of of (H)''? As far as I know, the fundamental matrix of a 2nd order ODE is defined as the fundamental matrix of the associated 1st order linear system. In other words, the fundamental matrix $phi(t)$ of $y''+py=0$ is not supposed to satisfy $phi''+pphi=0$, but rather $phi'=Aphi$, with $A$ the 2 by 2 matrix in your question.
$endgroup$
– Gil Bor
Jan 5 at 7:54
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%2f3057696%2ffundamental-matrix-of-hills-equation%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