Let y(t) be a nontrivial solution for the second order

Painevg

Painevg

Answered question

2021-12-18

Let y(t) be a nontrivial solution for the second order differential equation
x¨+a(t)x˙+b(t)x=0
to determine a solution that is linearly independent from y we set z(t)=y(t)v(t)
Show that this leads to a first order differential equation for v˙=w

Answer & Explanation

John Koga

John Koga

Beginner2021-12-19Added 33 answers

First of all, we say that y and z=yv are linearly independent if:
Ay+Bz=0A=B=0
Notice that:
Notice that:
Ay+By=y(A+Bv)
If v is constant for all t, then
A=BvAy+Byv=0
Therefore, we need that v is not constant over time in order to have that y and z are linearly independent.
Since y(t) is a solution of x¨+a(t)x˙+b(t)x=0, then:
y¨+a(t)y˙+b(t)y=0
Let's consider z(t)=y(t)v(t).If z(t) is a solution of x¨+a(t)x˙+b(t)x=0 then:
z¨+a(t)z˙+b(t)z=0
(y¨v)+a(t)(y˙v)+b(t)yv=0
y¨v+2y˙v˙+yv¨+a(t)(y˙v+yv˙)+b(t)yv=0
v(y¨+a(t)y˙+b(t)y)+2y˙v˙+a(t)yv˙=0
v0+2y˙v˙+yv¨+a(t)yv˙=0
yv¨+(2y˙+a(t)y)v˙=0
Now let's introduce w=v˙. The previous equation can be rewritten as:
{v˙=wyw˙=(2y˙+a(t)y)w
This means that the non constant function v satisfies the previous system of ODEs.

Kindlein6h

Kindlein6h

Beginner2021-12-20Added 27 answers

If y(t) is a known solution them substituting z(t)=λ(t)y(t) into the DE we have
yλ¨+(ay+2y˙)λ˙+y¨+ay˙+by=0
but
y¨+ay˙+by=0
then yλ¨+(ay+2y˙)λ{y}=0
so making λ=v we have
yv˙+(ay+2y˙)v=0
now solving for v after integrating we can obtain z=λy as a new independent solution.
RizerMix

RizerMix

Expert2021-12-29Added 656 answers

The instructions are given: set z=yv and obtain an equation in v˙, rewritten w
The substitution give
y¨v+2y˙w+w˙+ay˙v+ayw+byv=0
As y is a solution, a simplification occurs, and
2y˙w+w˙+ayw=0
In the latter, t and y are known.

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