Implicit numerical differentiation

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SUMMARY

The discussion centers on using the Crank–Nicolson method to solve a logistic function for population growth, specifically addressing the challenge of obtaining two solutions for the next time step, y(i+1), when solving a quadratic equation. The user inquires whether this indicates an error in their approach or if they can select the solution closest to the current value, y(i). The equation presented is a discretized form of the logistic growth model, where R represents the carrying capacity.

PREREQUISITES
  • Understanding of Crank–Nicolson numerical methods
  • Familiarity with quadratic equations and their solutions
  • Knowledge of logistic growth models in population dynamics
  • Basic proficiency in numerical analysis techniques
NEXT STEPS
  • Research the implications of multiple solutions in numerical methods
  • Study the stability and convergence of the Crank–Nicolson method
  • Explore alternative implicit schemes for solving differential equations
  • Learn about error analysis in numerical solutions of differential equations
USEFUL FOR

Mathematicians, computational scientists, and anyone involved in numerical modeling of population dynamics using implicit methods.

MaxManus
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Homework Statement



I am using Crank–Nicolson to solve a logistic function, modeling population growth.
To get the next time step, I have to solve a quadratic equation.
The problem is that i get two solutions for y(i+1). Does it mean that I am doing it wrong?
If not, can I just pick the solution that is closest to y(i)?
 
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Example:
Logistic growth, implicit scheme, not Crank-Nicolson.

[tex]\frac{u_{n+1}-u_n}{\Delta t} = n_{n+1}(1 - \frac{u_{n+1}}{R})[/tex]

R is the carrying capacity

Here I have to solve a quadratic equation to find [tex]u_{n+1}[/tex]
 

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