00:01
Okay, so we have a competitive situation here defined by these equations.
00:06
Our first point is to find the critical, our first step is to find the critical points.
00:13
So the critical points happen when the derivatives are equal to zero so that the right -hand sides are equal to zero.
00:23
And what we'll get is fixed values of s and r that satisfy both sets of equations.
00:38
The first equation tells us that s equals 0 or 10 minus 3s minus r equals 0, right? and then the second equation tells us r equals 0 or 7 minus s minus 4r equals 0.
01:07
So one obvious solution is r and s are both 0, which is kind of uninteresting physically, but nonetheless it's there.
01:23
There's another one that will have s equals 0 and then 7 minus 4r equals 0 so that r is 7 fourths and then we'll have another one that has r equals 0 and then 10 minus 3s which tells us that s is 10 thirds.
01:58
So these are about population, so fractional isn't necessarily bad.
02:04
And then our fourth solution has both of those.
02:11
So it's got to have 10 minus 3s minus r equals 0, and 7 minus s minus 4r equals 0.
02:25
So that's a pair of simultaneous equations.
02:28
So we get this.
02:33
So to discuss stability we have to look at the jacobian.
03:00
So what we do here, we're going to construct a matrix by taking the derivatives.
03:06
So let's think about the r and s derivatives of each of our functions.
03:14
So the first one, the r derivative of the first one is minus s.
03:21
The s derivative of the first one is 10 minus 3s minus r.
03:35
Actually, let's write it this way.
03:39
So it's 10 minus r minus 6s.
03:49
And then the r derivative of the second one is, that's this.
03:59
We're taking the r derivative.
04:01
So it's 7 minus s minus 8r.
04:08
And the s derivative of that is minus r.
04:25
Okay...