MATH201/content/Second order linear equations (lec 5-7).md

most of these "models" in EE are based on these DE. You'll see how important DE are in chemical, electrical, mechanical, engphys, civil (very important for civil!), (mining? idk what's in mining :D -prof) DE are important -prof

Second order linear equations

Second order equations arise from very simple problems many engineers face, for instance a pendulum can be described by a second order equation. #second_order

$a_{2}(t)y''+a_{1}(t)y'+a_{0}(t)y=f(t)$

To motivate our interest: #fix !Drawing 2023-09-15 13.32.48.excalidraw ma=my''=-by'-ky Look how a second order equation describes the motion of a mass-spring system!

Circuits that contains resistors, capacitors and inductors also behaves with this equation as well if you ignore the external magnetic fields around the circuit.

The equation my''+by'+ky=0 is a homogenous second order equation. (in this case, it's full name is homogenous second order linear equation with constant coefficients.)

Similar pattern with the electrical circuit analogy. This DE ignores external forces on the mass-spring system, it only considers the friction and the spring. If we push the mass then there would be an external force.

It's called second order because we have second derivative in the equation.

#ex #second_order

$y''-4y'+3y=0$

(This is obviously homogenous as stated by prof, although I don't understand why that is.) Imagine there's no y' (meaning no friction) you kind want the derivates to equal itself, an exponential! We guess the solution is of the form y(t)=e^{rt} y(t)=e^{rt} y'=re^{rt} y''=r^2e^{rt} r^2e^{rt}-4re^{rt}+3e^{rt}=0 <- Our guess worked! r^2-4r+3=0 ar^2+br+c=0 r_{1,2}=\frac{{-b\pm \sqrt{ b^2-4ac }}}{2a} so r_{1,2}=1,3 so two possibilities of the equation: y_{1}(t)=e^t or y_{2}(t)=e^{3t} so the general solution is the sum of the two possibilities (But why? See principle of super position below.)

y(t)=c_{1}e^{t}+c_{2}e^{3t}

and we're done.

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#end of lec 5 #start of lec 6

#ex #IVP #second_order Same equation from last lecture, but now an IVP:

y(t)=c_{1}e^{t}+c_{2}e^{3t} \quad c_{1},c_{2}\in\mathbb{R} \quad\text{let } y(0)=0,\ y'(0)=4\quad \text{ What is } c_{1}, c_{2}?

Lemma: y'(t)=c_{1}y_{1}+c_{2}y_{2} proof: let y_{1}=e^{r_{1}t}\qquad y_{2}=e^{r_{2}t} y(t)=c_{1}e^t+c_{2}e^{3t} y'(t)=c_{1}r_{1}e^{r_{1}t}+r_{2}c_{2}e^{r_{2}t} y'(t)=c_{1}r_{1}e^{r_{1}t}+c_{2}r_{2}e^{r_{2}t} since c_{1}r_{1} is just a product of two arbitrary constants, we can replace them with a new constant. y'(t)=c_{1}e^{r_{1}t}+c_{2}e^{r_{2}t} y'(t)=c_{1}y_{1}+c_{2}y_{2} \quad \Box

We are given y(0)=0 c_{1}e^0+c_{2}e^{3*0}=0 c_{1}+c_{2}=0 We are also given y'(0)=4 c_{1}+3c_{2}=4

Solving the linear system of equations gives: c_{1}=-2,\ c_{2}=2 which gives the solution:

y'(t)=-2e^t+2e^{3t}

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Remember from the example above where I said the "general solution is the sum of the two possibilities"? Let's explore and see why that is: Recap: suppose we have an equation of the form ay''+by'+cy=0 y(t)=e^{rt} then ar^2+br+c=0 case i) r_{1},r_{2}=\frac{{-b\pm \sqrt{ b^2-4ac }}}{2a}, {r_{1}}\ne r_{2} y_{1}(t)=e^{r_{1}t}\qquad y_{2}(t)=e^{r_{2}t} y(t)=c_{1}e^{r_{1}t}+c_{2}e^{r_{2}t} is also a solution. But why? Principle of super position.

Principle of super position:

If y_{1}(t) solves ay''+by'+cy=f_{1}(t) and y_{2}(t) solves ay''+by'+cy=f_{2}(t) on an interval I. Then the following function that is a combination: y(t)=c_{1}y_{1}(t)+c_{2}y_{2}(t) solves ay''+by'+cy=c_{1}f_{1}(t)+c_{2}f_{2}(t) Now we prove it: Plugging in y=c_{1}y_{1}(t)+c_{2}y_{2}(t) into ay''+by'+cy=c_{1}f_{1}(t)+c_{2}f_{2}(t) gives us: a(c_1y_{1}''+c_{2}y_{2}'')+b(c_1y_{1}'+c_{2}y_{2}')+c(c_1y_{1}+c_{2}y_{2})=c_{1}f_{1}(t)+c_{2}f_{2}(t) moving terms gives us: c_{1}(ay_{1}''+by_{1}'+cy_{1})+c_{2}(ay_{2}''+by_{2}'+cy_{2})=c_{1}f_{1}(t)+c_{2}f_{2}(t) \quad \Box

Okay but none of that makes sense, how do we use the proof? Let the following: y_{1}(t)=e^{r_{1}t} solves ay''+by'+cy=0 y_2(t)=e^{r_{2}t} solves ay''+by'+cy=0 f_{1}(t)=f_{2}(t)=0 This following can be concluded: y(t)=c_{1}e^{r_{1}t}+c_{2}e^{r_{2}t} must solve ay''+by'+cy=0 by principle of super position.

Yay! Note this is only true when f_{1}(t)=f_{2}(t)=0 aka your RHS in the second order equation must be 0.

case ii) r_{1}=r_{2}=\frac{-b}{2a} if b^2-4ac=0 if we assume y_{1}=e^{r_{1}t}, y_{2}=e^{r_{1}t} like before then we get: y(t)=c_{1}e^{r_{1}t}+c_{2}e^{r_{1}t}=ce^{r_{1}t} <- this doesn't seem like it works! We need two integration constants for a second order equation. y_{1}(t)=e^{-bt/2a}, y_{2}(t)=te^{-bt/2a} for time being we take this as true, we can prove it later. y(t)=c_{1}e^{-\frac{bt}{2a}}+c_{2}te^{-\frac{bt}{2a}} we can check later at home, but also, how was the idea for this found? He will tell us later.

linear algebra 101: linear independence makes unit vectors, which forms a basis.

definition: if y_{1}, y_{2} are solutions to a(t)y''+b(t)y+c(t)=0 on some interval I_{1} then they are called linearly independent if none of them is a constant multiple of the other. Theorem: If y_{1}(t), y_{2}(t) are linearly independent solutions to ay''+by'+cy=0 then any other solution can be written as y(t)=c_{1}y_{1}(t)+c_{2}y_{2}(t)

how do we know the two solutions are linearly independent? Test for linear independence: y_{1}, y_{2} are solutions to a(t)y''+b(t)y+c(t)y=0 on some interval I_{1} then they are called linearly independent iff W(y_{1},y_{2})(t)=\det \begin{pmatrix}y_{1} & y_{2} \\y_{1}' & y_{2}'\end{pmatrix}\ne 0 (i) b^2-4ac>0 \Rightarrow r_{1}\ne r_{2} y_{1}=e^{r_{1}t}, y_{2}=e^{r_{2}}t W(y_{1},y_{2})=\det\begin{pmatrix}e^{r_{1}t} & e^{r_{2}t} \\r_{1}e^{r_{1}t} & r_{2}e^{r_{2}t}\end{pmatrix} =e^{t(r_{1}+r_{2})}(r_{2}-r_{1})\ne 0 (ii) b^2-4ac=0\Rightarrow r_{1}=r_{2}=-\frac{b}{2a}=r y_{1}(t)=e^{rt}, y_{2}(t)=te^{rt} W(y_{1},y_{2})=\det\begin{pmatrix}e^{rt} & te^{rt} \\re^{rt} & e^{rt}(1+rt)\end{pmatrix} =e^{2rt}(1+rt)-rte^{rt}e^{2rt} =e^{2rt}\ne 0

#ex #IVP #second_order

y''-2y'+y=0, y(0)=1, y'(0)=0

e^{rt}(\underset{ = }{ r^2-2r+1 })=0 (r-1)^2=0\Rightarrow r_{1}=r_{2}=1 y(t)=c_{1}e^t+c_{2}te^t y(0)=c_{1}=1 y(t)=e^t+c_{2}te^t y'(0)=0=1+c_{2}\Rightarrow c_{2}=1

y(t)=e^t-te^t

Wow, that was a lot today, my notes look like a mess. I'll have to clean this up and understand what's going on later. #end of lecture 6

#start of lecture 7 (sept 20) from last class: if r_{1}=r_{2}=-\frac{b}{2a}=r y_{1}=e^{rt}, \quad y_{2}=te^{rt} general solution:

y(t)=c_{1}y_{1}(t)+c_{2}y_{2}(t)

however there's a third option: b^2-4ac<0 then we have complex roots r_{1,2}=-\frac{b}{2a}\pm\frac{i\sqrt{ 4ac-b^2 }}{2a}=\alpha+i\beta <- Complex conjugates. And due to fundamental theorem of algebra, there are only 2 roots.

e^{r_{1}t}=e^{(\alpha+i\beta)t}=e^{\alpha t}+e^{i\beta t} side note: there are no numbers that are more than two components that are "useful", even quaternions e^{i\beta t}=e^{i\theta} expand into power series: =1+\frac{i\theta}{1!}+\frac{{(i\theta)^2}}{2!}=\frac{{(i\theta)^3}}{3!}\dots =1+\frac{i\theta}{1!}-\frac{\theta^2}{2!}-\frac{i\theta^3}{3!}+\frac{\theta^4}{4!}+\frac{i\theta^5}{5!}+\dots =\left( 1-\frac{\theta^2}{2!}+\frac{\theta^4}{4!}-\dots \right)+i\left( \frac{\theta}{1!}-\frac{\theta^3}{2!}+\frac{\theta^5}{3!}\dots\right) e^{i\theta}=\cos(\theta)+i\sin(\theta) \quad \Box We have proven the Euler formula

y(t)=e^{rt}=e^{\alpha t}(\cos \beta t+i\sin \beta t) Lemma: If u(t)+iv(t) solves ay''+by'+cy=0 then u(t),\ v(t) are also solutions. Proof: a(u+iv)''+b(u+iv)'+c(u+iv)=0 \underbrace{ { (au''+bu'+cu) } }_{ =0 }+i\underbrace{ (av''+bv'+cv) }_{ =0 }=0 y_{1}(t)=e^{\alpha t}\cos(\beta t),\ y_{2}(t)=e^{\alpha t}\sin(\beta t) \alpha=-\frac{b}{2a},\quad \beta={\frac{\sqrt{ 4ac-b^2 }}{2a}} y(t)=c_{1}y_{1}+c_{2}y_{2} now we have to test the two solutions are linearly independent W[y_{1},y_{2}]=\det\begin{pmatrix}y_{1} & y_{2} \\ y_{1}' & y_{2}'\end{pmatrix}\ne0 <- remember to do/show this at home

#ex #IVP

y''-2y'+5y=0 \quad y(0)=0 \quad y'(0)=2

r^2-2r+5=0<-characteristic equation r_{1,2}={1\pm \frac{\sqrt{ -4b }}{2}}=1\pm2i y_{1}=e^t\cos(2t) y_{2}=e^t\sin(2t) general solution: y(t)=e^{\alpha t}(c_{1}\cancel{ \cos 2 t }+c_{2}\sin 2 t) y(0)=0=c_{1} y'(0)=c_{2}(e^t) I missed stuff here that he erased general solution is:

y(t)=e^t\sin(2t)

it has a nice graph, where if it was a circuit it would blow up or if it was a bridge it would collapse

Something more difficult now:

ay''+by'+cy=f(t) Again, a mass-spring system without any external force. if f(t)=0 we can find the solution easily and use superposition to get the general solution ay''+by'+cy=0 -> general solution is y(t)=c_{1}y_{1}(t)+c_{2}y_{2}(t)

If we can find just one solution in ay''+by'+cy=f(t) let it be y_{p}(t) then the sum of the solutions y(t)=c_{1}y_{1}(t)+c_{2}y_{2}(t)+y_{p}(t) must solve ay''+by'+cy=f(t)

Theorem: If a(t),\ b(t),\ c(t) are continuous on I , then IVP: a(t)y''+b(t)y'+c(t)y=f(t) ; y(t_{o})=y_{o} \ , y'(t_{o})=y_{1} has a unique solution. we will do the proofs next class. #end of lecture 7