Why Is y''(a) Determined by y(a) and y'(a) in a Differential Equation?

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Discussion Overview

The discussion revolves around the determination of the second derivative y''(a) in the context of nth order linear differential equations, specifically focusing on the relationship between initial conditions and the solution of such equations. It includes theoretical aspects and mathematical reasoning related to differential equations.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • Some participants propose that in a linear differential equation of the form y'' + py' + qy = 0, the value of y''(a) can be expressed in terms of y(a) and y'(a) through the equation y''(a) = -py'(a) - qy(a).
  • Another participant suggests that the general solution of the differential equation can be represented as a linear combination of two linearly independent solutions, which allows for the determination of constants based on initial conditions.
  • One participant questions the dimensionality of the solution set for a second-order system, implying a need for clarification on this aspect.
  • A later reply dismisses the need for further explanation on the relationship between y''(a) and the initial conditions, suggesting that the reasoning is straightforward.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of explaining the relationship between y''(a) and the initial conditions, with some asserting clarity while others seek deeper understanding. The discussion does not reach a consensus on the need for further elaboration.

Contextual Notes

There are assumptions regarding the linearity of the differential equation and the independence of the solutions that are not explicitly stated. The discussion also does not resolve the implications of the dimensionality of the solution set.

AndreaA
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Indicate why we can impose only n initial conditions on a solution of nth order linear differential equation.

A) Given the equation y'' + py'+ qy = 0
explain why the value of y''(a) is determined by the values of y(a) and y'(a).

B) Prove that the equation y'' - 2y' -5y =0
has the solution satisfying the conditions y(0) = 1, y'(0) = 0, and y''(0) = C
if and only if C = 5.
 
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AndreaA said:
A) Given the equation y'' + py'+ qy = 0
explain why the value of y''(a) is determined by the values of y(a) and y'(a).

The DE is linear so it must have a general solution which is a linear combination of two linearly independent solutions y1(x) and y2(x).

y(x)=c1y1(x) + c2y2(x) .

c1 and c2 can be determined uniquely from the given initial conditions. So the result can be deduce from here.
 


Do you need to the dimension of the solution set to a second order system is two dimensional?
 


AndreaA said:
A) Given the equation y'' + py'+ qy = 0
explain why the value of y''(a) is determined by the values of y(a) and y'(a).

Because y''(a) = -py'(a) - qy(a).

Move along, please, there's nothing to explain here...
 

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