Find Critical Points of x' and y': ax - bxy & bxy - cy

In summary: Equilibrium solutions are points where the variables are equal, while singular points are points where the variables are not equal.
  • #1
rad0786
188
0

Homework Statement



Find the singular points for:

x' = ax - bxy
y' = bxy - cy


Homework Equations





The Attempt at a Solution



ax - bxy = 0
bxy - cy = 0

implies

x = 0 or y = a/b

y = 0 or x = c/b

the the critical points are (o,o) or (c/b, a/b)

why is (0, a/b) and (c/b, 0) NOT a critical point??
 
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  • #2
why is (0, a/b) and (c/b, 0) NOT a critical point??

Because, [itex]x'[/itex] and [itex]y'[/itex] are not 0 for those values of x and y. Substitute them to check.
 
  • #3
Why would you think they would be? They clearly don't satisfy the equations you give.

When you have a system of non-linear equations with several different solutions, the x and y are not independent- you can't just combine any value of x with any value of y: it is the specific x, y pair that satisfies the equations.
 
  • #4
Oh I see my mistake.

I thought that they were dependent.
 
  • #5
I have been working with the equations:

x' = rx - sxy/(1+tx)

y' = sxy/(1+tx) - wy


I find that the only singular point is (0,0)

is this correct?


I got another point, ( w/(s-w) , r/s ), however, that cannot be a critical point because when substituting, x' and y' fail to be 0.
 
  • #6
rad0786 said:
I have been working with the equations:

x' = rx - sxy/(1+tx)

y' = sxy/(1+tx) - wy


I find that the only singular point is (0,0)

is this correct
I got another point, ( w/(s-w) , r/s ), however, that cannot be a critical point because when substituting, x' and y' fail to be 0.
Since (w/(s-w), r/s) does not satisfy the equations for a critical point, how did you get that?

The equations for a critical point are x'= rx- sxy/(1+ tx)= 0 and y'= sxy/(1+ tx)- wy= 0. The first gives sxy/(1+ tx)= rx and the second sxy/(1+ tx)= wy. Since the left sides of both equations are the same, rx= wy. Every point on the line y= (r/w)x (or, if w= 0, the line x= 0) is a singular point.
 
  • #7
HallsofIvy said:
Since (w/(s-w), r/s) does not satisfy the equations for a critical point, how did you get that?

x'= rx- sxy/(1+ tx)= 0

x(r-sy/(1 + tx)) = 0

x= 0 and (r-sy/(1 + tx)) = 0

since x = 0 r-sy = 0

therefore, y = r/s


similarly we do that for y'= sxy/(1+ tx)- wy= 0 and we get y = 0 and x = w/(s-w)


HallsofIvy said:
The equations for a critical point are x'= rx- sxy/(1+ tx)= 0 and y'= sxy/(1+ tx)- wy= 0. The first gives sxy/(1+ tx)= rx and the second sxy/(1+ tx)= wy. Since the left sides of both equations are the same, rx= wy. Every point on the line y= (r/w)x (or, if w= 0, the line x= 0) is a singular point.


when I substitute y= (r/w)x into x'= rx- sxy/(1+ tx), x' is NOT zero. So how could this line be a line of singular points?
 
  • #8
also, what are the differences between equilibrium solutions and singular points?
 

1. What are critical points in a function?

Critical points in a function refer to the points where the function's derivative is equal to zero or does not exist. These points are important in determining the behavior and shape of a function.

2. How do I find critical points of a function?

To find the critical points of a function, you need to first take the derivative of the function with respect to the variable in question. Then, set the derivative equal to zero and solve for the variable to find the critical points.

3. What is the relationship between critical points and extrema?

Critical points are directly related to extrema, which refer to the maximum or minimum values of a function. In fact, critical points are often used to help find the location of extrema in a function.

4. How do I determine if a critical point is a maximum or minimum?

In order to determine if a critical point is a maximum or minimum, you can use the second derivative test. If the second derivative is positive at the critical point, then it is a minimum. If the second derivative is negative, then it is a maximum.

5. Can a function have more than one critical point?

Yes, a function can have multiple critical points. In fact, a polynomial function of degree n can have up to n-1 critical points. It is important to consider all critical points when analyzing the behavior of a function.

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