# Classification of steady states

1. May 15, 2012

### sid9221

http://dl.dropbox.com/u/33103477/harvesting.png [Broken]

So am I right in saying the 2 steady states are:

$$N_1=\frac{h}{r}, N_2=\frac{1-h}{\alpha}$$

Now plugging in N_1 into the equation I get:

$$\frac{-h^2 \alpha}{r} < 0$$

So N_1 is stable.

But I can't quite figure out how to classify N_2

Any advice ?

Last edited by a moderator: May 6, 2017
2. May 15, 2012

### vela

Staff Emeritus
No, that's not correct. In steady state, the population isn't changing; that is, it's when dN/dt=0.

3. May 15, 2012

### sid9221

Exactly what I was thinking, I was shying away from doing that as they did not look "pretty" and I do not know what a "harvesting term" is.

So the steady states should be:

$$N_1 = \frac{r-\sqrt{r(r-4\alpha h)}}{2ar}, N_2 = \frac{r+\sqrt{r(r-4\alpha h)}}{2ar}$$

N_1<N_2

?

4. May 15, 2012

### vela

Staff Emeritus
Yes, those are right.

The harvesting term is h. It could represent, say, the number of deer killed by hunting every year. The population is decreased by that amount every year.

5. May 15, 2012

### sid9221

How do I classify these as if I sub them back into the differential equation I get 0 ??

6. May 15, 2012

### vela

Staff Emeritus
You need to find the correct criterion. You're trying to figure out what happens if the system is disturbed slightly from an equilibrium point. Will it move back toward that point or will it move away? It might help to plot the function dN/dt as a function of N.

7. May 17, 2012

### spamiam

Hi sid9221,

Out of curiosity, did this question come from a textbook? James Murray has a good book on mathematical biology and parts are available on Google books. He discusses of linear stability here.

vela is exactly right, you need to consider the effect of a small perturbation at the steady state. In the end, all you really need to do is set $f(N) = \frac{d N}{dt}$ and consider $f'(N)$ evaluated at the steady states.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook