# Find the approximate linear ODE system

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1. Nov 9, 2015

### NiallBucks

dx/dt = x-y^2 dy/dt= x^2 -xy -2x
For each critical point, find the approximate linear OD system that is valid in a small neighborhood of it.

I found the critical points which are (0,0),(4,2),(4,-2) but have no idea how to do the above question! please help!

2. Nov 9, 2015

### Krylov

Just calculate the Jacobian matrices of the vector field at the three critical points. Your approximate linear ODE system at the critical point $(x_c,y_c)$. is going to be of the form $\dot{u}(t) = A(x_c,y_c)u(t)$ where $A(x_c,y_c)$ is the Jacobian matrix.

To check that this system is valid in a small neighborhood, verify that $A(x_c,y_c)$ has no spectrum on the imaginary axis. Which theorem do you use here?

3. Nov 9, 2015

### NiallBucks

I'm not sure about the theorem

4. Nov 9, 2015

### Krylov

It's the Hartman–Grobman theorem. You should consider looking it up, it's worthwhile.

5. Nov 9, 2015

### HallsofIvy

Staff Emeritus
To "linearize" an equation simply means to replace any non-linear function by a linear approximation. But the only linear approximations to $x^2$ and $xy$ are "0". At (0, 0), dx/dt= x- y^2 linearizes to dx/dt= x and dy/dt= x^2- xy- 2x to dy/dt= -2x.

About (4, 2), let u= x- 4 and v= y- 2 so that x= u+ 4, y= v+ 2, dx/dt= du/dt, and dy/dt= dv/dt. The equations become du/dt= u+4- (v+2)^2= u+ 4- v^2- 4v- 4 which linearizes to du/dt= u- 4v and dv/dt= (u+ 4)^2- (u+ 4)(v+ 2)- 2(u+ 4)= u^2+ 8u+ 16- uv- 4v- 2u- 8- 2u- 8= u^2- uv+ 4u- 6v- uv which linearizes to dv/dt= 4u- 6v.

6. Nov 9, 2015

### Krylov

Why (and when, and when not) the linearisation says something meaningful about the original system in a neighborhood of the critical point, is less simple, though.

7. Dec 3, 2015

### Staff: Mentor

(4, -2) is NOT a critical point.

Also, you posted a question in another forum section about the critical points of this system. Your other question was posted in the right section (Calculus & Beyond under Homework & Coursework Questions). Please take care to post homework questions there, not here in the technical math sections.

Also, don't post essentially the same question in multiple forum sections.