Determine if a solution to a differential equation

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SUMMARY

This discussion focuses on determining the periodicity of solutions to differential equations without explicit solutions. It establishes that solutions to linear differential equations with constant coefficients are periodic if their characteristic values are imaginary. The example provided is the equation y" + a²y = 0, which yields periodic solutions such as cos(ax) and sin(ax). Additionally, the conversation touches on generating differential equations with periodic solutions, highlighting the role of characteristic equations.

PREREQUISITES
  • Understanding of differential equations, specifically linear differential equations with constant coefficients.
  • Familiarity with characteristic equations and their role in determining solution behavior.
  • Knowledge of periodic functions, particularly sine and cosine functions.
  • Experience with mathematical software like Maple for graphing and analyzing differential equations.
NEXT STEPS
  • Research the properties of linear differential equations with constant coefficients.
  • Explore the derivation and implications of characteristic equations in differential equations.
  • Learn about the methods for generating differential equations with known periodic solutions.
  • Investigate the use of Maple for solving and graphing differential equations, focusing on parameter variations.
USEFUL FOR

Mathematicians, students studying differential equations, and anyone interested in the analytical methods for determining periodicity in solutions to differential equations.

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Is it possible to determine if a solution to a differential equation is or isn't periodic, even if you don't know the solution explicitly? Also, is it possible to generate differential equations that have periodic solutions (besides the obvious ones like the solution to y" = -ay)? The reason why I'm asking is that I was fooling around with graphing y" = y(y-10)(50-y) in Maple (for y(0) = 20, y'(0)=0), but depending on what the parameters of the graph are I seem to get different answers. I'm just wondering if there's an analytical way to determine periodicity.
 
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To give a simple, special case of your question: the solutions to a linear differential equation with constant coefficients are periodic if and only if the characteristic values are imaginary.

For any a, ai is pure imaginary and has conjugate -ai. (r-ai)(r+ai)= r2+a2= 0 has those as as solutions and so the differential equation y"+ a2y= 0 has solutions cos(ax) and sin(ax). If you want to "fancy" it up a bit more add some other factors say (x- b)(x2+a2= 0 is the characteristic equation for a d.e. that has solutions ebx as well as sin(ax) and cos(ax).
 

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