I need to solve the following differential equation x^2y''+(ax-b)y'-ay=0 with a,b>0,x\geq0 and

veksetz

veksetz

Answered question

2021-12-17

I need to solve the following differential equation
x2y+(axb)yay=0
with a,b>0,x0 and y(0)=0. The power series method will fail since there is a singularity at x=0, while the form of the equation does not conform with the Frobenius method.

Answer & Explanation

Kindlein6h

Kindlein6h

Beginner2021-12-18Added 27 answers

x2y+(axb)yay=0
Start by removing the leading behavior for small x. Near x=0 the leading terms in the equation are
byay0
(this is valid as long as in the end, as x0,y does not grow as fast as yx2 or yx2, does not grow as fast yx).
The solution to the approximate equation is y=eabx which motivates the substitution
y=u(x)eabx
y=ueabxabueabx
y=ueabx2abueabx+a2b2ueabx
and the differential equation becomes
x2u(2abx2ax+b)u+(a2b2x2a2bx+aa)u=0
with u(0)=0
Now if we let u(x)=0ukxk
0k(k1)ukxk2ab0kukxk+1+a0kukxk-bkukxk1+a2b20ukxk+2a2b0ukxk+1=0
And in each term we can substitute an offset index to always get xk this gives
[k(k1)+ak)uk[2ab(k

Gerald Lopez

Gerald Lopez

Beginner2021-12-19Added 29 answers

It has a trivial solution y=axb
Let y=(axb)u
Then y=(axb)u+au
y=(axb)u+au+au=(axb)u+2au
x2((axb)u+2au)+(axb)((axb)u+au)a(axb)u=0
x2(axb)u+2ax2u+(axb)2u+a(axb)ua(axb)u=0
x2(axb)u=(2ax2+(axb)2)u
uu=2aaxbax+bx2
lnu=2ln(axb)alnxbx+c
u=Cebxxa(axb)2
u=C1+C2xebxxa(axb)2dx
y=C1(axb)+C2(axb)
xebxxa(axb)2dx
RizerMix

RizerMix

Expert2021-12-29Added 656 answers

With Laplace transforms this equation will be
s2F(s)+(4ab)sF(s)+2(1a)F(s)=0
which is Euler's equation.

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