 # Nullcline: Definition, Examples & Graph

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A nullcline is a point where a vector field changes direction . The name means zero (null) slope (cline).

## Nullcline in Differential Equations

In an autonomous system of two differential equations They can be defined as [2, 3]:

• x-nullcline: The set of points in the phase plane where dx/dt = 0. Geometrically, the vectors at these points are vertical (straight up and down). You can find the x-nullclines by solving f(x, y) = 0.
• y-nullcline: The set of points in the phase plane where dy/dt = 0. Geometrically, the vectors at these points are horizontal (moving to the left or right). You can find the x-nullclines by solving g(x, y) = 0.

If you have a different variable in your differential equations, the nullcline is called by that variable. For example, if you have dR/dt and dL/dt, then the curve where dR/dt = 0 is called the R null cline and the curve where dL/dt = 0 is called the L null cline.

## Nullcline and Sketching Phase Planes

Nullclines are very useful for analyzing and sketching phase planes. The x-nullcline divides the phase plane into two regions where x moves in opposite directions. Similarly, the y-nullcline divides the phase plane into two areas where y either increases or decreases. Together, the x- and y-nullclines divide the phase plane into four regions :

1. x and y increase
2. x and y decrease
3. x increases and y decreases
4. y increases and x decreases.

In the 2D plane, every point where the nullclines intersect is an equilibrium point.

## Example

Example question: Find the x- and y- nullclines and the equilibrium point(s) for the following system of differential equations and sketch the result: The nullclines happen when x′ = 0 or y′ = 0. Therefore, solve both equations for zero to get the nullclines: The following graph shows the solution: The vectors along the x-nullcline are vertical and along the y-nullcline they are horizontal; when a nullcline passes through the equilibrium point (-1, 1), the direction is reversed.

Although the nullclines are represented by lines in this example, they could technically be any curve.

## References

 Izhikevich, E. (2007). Dynamical Systems in Neuroscience. MIT Press.
 Duke U. (2000). Systems of Differential Equations: Models of Species Interaction. Retrieved July 22, 2021 from: https://services.math.duke.edu/education/postcalc/predprey/pred3b.html
 A quick guide to sketching phase planes. Retrieved July 22, 2021 from: https://mcb.berkeley.edu/courses/mcb137/exercises/Nullclines.pdf

CITE THIS AS:
Stephanie Glen. "Nullcline: Definition, Examples & Graph" From CalculusHowTo.com: Calculus for the rest of us! https://www.calculushowto.com/nullcline-definition-examples-graph/
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