In the last video I presented two examples
of a particular solution on a phase plane

going from X and Y of t to an X, Y plot.

In this video, I'll discuss properties
of the phase plane

a little bit more generally.

Here are the differential equations
we're working with.

The rate of change of X is a function
of X and Y, and the rate of change of

Y is a function of X and Y.

So, to use the language we've used before,
I would say that X and Y determine the

rate of change of X and Y and
thus X and Y themselves.

So as with all the dynamical systems we've
looked at, this is a deterministic

dynamical system.

X and Y determines the future--the present
value of X and Y---determine

the future values of X and Y
through this relationship.

So let's think about what that would
look like geometrically.

So suppose we are somewhere here, at a
particular X, Y point,

So I could say where am I going next?
Where will I go next in terms of X and Y?

Well, I ask the equation--the equation
determines the next values through this

relationship and I could think of dX/dt as
telling me how much and the direction of

X change - does it go up or down, which
in this case, i.e. does X increase or

decrease, which means going this way,
or that way.

Similarly Y can increase or decrease
meaning going this way or that way.

So I can think, if I am at this point, I
plug into here I get these two and I

can picture this as giving me an arrow,
a direction. It tells me where to move,

where do X and Y go next.

So I can imagine, maybe that's a little
arrow, that's my direction.

This little device tells me "Go in this
direction at a certain speed."

Then I could be at another point and I
plug that in here and say, "what are my

directions, where am I going,
what is my rate of change?"

That might give me a different arrow
over here or I might have something

different altogether.

So every point in space has a set of
directions associated with it

given by these equations.

So knowing the value of a current point
determines all the successive points

that this goes to.

One could picture this if you want as a
whole bunch of arrows, technically a

vector field, here on this plane and that
a particular trajectory or solution would

follow the vector field like a particle
drifting through a fluid

moving with a speed.

So this might give a trajectory
something like that.

So, again, the main point is that X and Y
determine subsequent values of

X and Y. This is deterministic.

So this fact of determinism, has an
important geometric consequence.

In particular, it tells us that two curves
in phase space - let's draw some axes

on again to be clear - there's an X,
there's a Y - it says that two solutions

in phase space can never cross.

And here's why: assume there is a crossing
point. I'll draw that there in red.

What this equation says is: "if you tell
me X and Y (f is a deterministic function,

g is a deterministic function)"if you tell
me X and Y, there is one and only one

dX/dt and dY/dt." That means there is one
and only one arrow indicating the

direction of change from this point.

But if we have two curves crossing like
this, that violates this condition.

If this was possible, then there would
be two possible direction curves

from this red dot. This would be
ambiguous and non-deterministic.

But that's not the case. 
The equations are deterministic.

So this scenario here cannot happen
and the conclusion is that in the phase

plane trajectories, i.e. things like this,
cannot cross.

So we can have all sorts of other
behaviors; it can loop around, it can

cycle, it can go to a fixed point, 
it can go off to infinity, but the only

rule is that trajectories can't cross.

In the next video, I'll present a few more
examples and we'll see one of the

important consequences of this fact.