Frobenius Method to solve $x(1 - x)y'' - 3xy' - y = 0$
$begingroup$
So, Im trying to self-learn method of frobenius, and I would like to ask if someone can explain to me how can we solve the following DE about $ x = 0$ using this method.
$$ x(1 - x)y'' - 3xy' - y = 0 $$
ordinary-differential-equations power-series frobenius-method
$endgroup$
add a comment |
$begingroup$
So, Im trying to self-learn method of frobenius, and I would like to ask if someone can explain to me how can we solve the following DE about $ x = 0$ using this method.
$$ x(1 - x)y'' - 3xy' - y = 0 $$
ordinary-differential-equations power-series frobenius-method
$endgroup$
add a comment |
$begingroup$
So, Im trying to self-learn method of frobenius, and I would like to ask if someone can explain to me how can we solve the following DE about $ x = 0$ using this method.
$$ x(1 - x)y'' - 3xy' - y = 0 $$
ordinary-differential-equations power-series frobenius-method
$endgroup$
So, Im trying to self-learn method of frobenius, and I would like to ask if someone can explain to me how can we solve the following DE about $ x = 0$ using this method.
$$ x(1 - x)y'' - 3xy' - y = 0 $$
ordinary-differential-equations power-series frobenius-method
ordinary-differential-equations power-series frobenius-method
edited Nov 30 '16 at 0:17
user137731
asked Oct 28 '12 at 5:04
ILoveMathILoveMath
1
1
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
The motivation behind Frobenius method is to seek a power series solution to ordinary differential equations.
Let $y(x) = displaystyle sum_{n=0}^{infty} a_n x^n$. Then we get that $$y'(x) = sum_{n=0}^{infty} na_n x^{n-1}$$
$$3xy'(x) = sum_{n=0}^{infty} 3na_n x^{n}$$
$$y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-2}$$
$$xy''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-1} = sum_{n=0}^{infty} n(n+1)a_{n+1} x^{n}$$
$$x^2y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n}$$
The ODE is $$xy'' - x^2 y'' -3xy' - y = 0$$
Plugging in the appropriate series expansions, we get that
$$sum_{n=0}^{infty} left(n(n+1)a_{n+1} - n(n-1)a_n - 3na_n - a_nright)x^n = 0$$
Hence, we get that
$$n(n+1)a_{n+1} = (n(n-1) +3n+1)a_n = (n+1)^2 a_n implies a_{n+1} = dfrac{n+1}{n}a_n$$
First note that $a_0 = 0$. Choose $a_1$ arbitrarily. Then we get that $a_2 = 2a_1$, $a_3 = 3a_1$, $a_4 = 4a_1$ and in general, $a_{n} = na_1$.
Hence, the solution is given by
$$y(x) = a_1 left(x+2x^2 + 3x^3 + cdotsright)$$
This power series is valid only within $vert x vert <1$. In this region, we can simplify the power series to get
begin{align}
y(x) & = a_1 x left(1 + 2x + 3x^2 + cdots right)\
& = a_1 x dfrac{d}{dx} left(x + x^2 + x^3 + cdots right)\
& = a_1 x dfrac{d}{dx} left(dfrac{x}{1-x}right)\
& = a_1 dfrac{x}{(1-x)^2}
end{align}
$endgroup$
add a comment |
$begingroup$
The order reduction method seeks a second basis solution in the form $y=y_1u$, where $y_1(x)=frac{x}{(1-x)^2}$ is the already found basis solution.
$$
x(1-x)[y_1u''+2y_1'u']-3x[y_1u']=0
implies frac{u''}{u'}=frac{3y_1-2(1-x)y_1'}{(1-x)y_1}
$$
Insert $y_1(x)=dfrac1{(1-x)^2}-dfrac1{1-x}$, $y_1'=dfrac2{(1-x)^3}-dfrac1{(1-x)^2}$, $y_1''=dfrac{6}{(1-x)^4}-dfrac{2}{(1-x)^3}$
into that formula to find
begin{align}
frac{u''}{u'}&=frac{-frac1{(1-x)^2}-frac1{1-x}}{frac{x}{1-x}}=-frac{2-x}{x(1-x)}=-frac{2}x+frac1{1-x}
\
implies u'&=frac1{x^2(1-x)}=frac{1+x}{x^2}-frac1{1-x}
\
implies u&=-frac1x+ln|x(1-x)|
end{align}
so that the second basis solution is
$$
y_2=frac{xln|x(1-x)|-1}{(1-x)^2}
$$
$endgroup$
add a comment |
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2 Answers
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2 Answers
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$begingroup$
The motivation behind Frobenius method is to seek a power series solution to ordinary differential equations.
Let $y(x) = displaystyle sum_{n=0}^{infty} a_n x^n$. Then we get that $$y'(x) = sum_{n=0}^{infty} na_n x^{n-1}$$
$$3xy'(x) = sum_{n=0}^{infty} 3na_n x^{n}$$
$$y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-2}$$
$$xy''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-1} = sum_{n=0}^{infty} n(n+1)a_{n+1} x^{n}$$
$$x^2y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n}$$
The ODE is $$xy'' - x^2 y'' -3xy' - y = 0$$
Plugging in the appropriate series expansions, we get that
$$sum_{n=0}^{infty} left(n(n+1)a_{n+1} - n(n-1)a_n - 3na_n - a_nright)x^n = 0$$
Hence, we get that
$$n(n+1)a_{n+1} = (n(n-1) +3n+1)a_n = (n+1)^2 a_n implies a_{n+1} = dfrac{n+1}{n}a_n$$
First note that $a_0 = 0$. Choose $a_1$ arbitrarily. Then we get that $a_2 = 2a_1$, $a_3 = 3a_1$, $a_4 = 4a_1$ and in general, $a_{n} = na_1$.
Hence, the solution is given by
$$y(x) = a_1 left(x+2x^2 + 3x^3 + cdotsright)$$
This power series is valid only within $vert x vert <1$. In this region, we can simplify the power series to get
begin{align}
y(x) & = a_1 x left(1 + 2x + 3x^2 + cdots right)\
& = a_1 x dfrac{d}{dx} left(x + x^2 + x^3 + cdots right)\
& = a_1 x dfrac{d}{dx} left(dfrac{x}{1-x}right)\
& = a_1 dfrac{x}{(1-x)^2}
end{align}
$endgroup$
add a comment |
$begingroup$
The motivation behind Frobenius method is to seek a power series solution to ordinary differential equations.
Let $y(x) = displaystyle sum_{n=0}^{infty} a_n x^n$. Then we get that $$y'(x) = sum_{n=0}^{infty} na_n x^{n-1}$$
$$3xy'(x) = sum_{n=0}^{infty} 3na_n x^{n}$$
$$y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-2}$$
$$xy''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-1} = sum_{n=0}^{infty} n(n+1)a_{n+1} x^{n}$$
$$x^2y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n}$$
The ODE is $$xy'' - x^2 y'' -3xy' - y = 0$$
Plugging in the appropriate series expansions, we get that
$$sum_{n=0}^{infty} left(n(n+1)a_{n+1} - n(n-1)a_n - 3na_n - a_nright)x^n = 0$$
Hence, we get that
$$n(n+1)a_{n+1} = (n(n-1) +3n+1)a_n = (n+1)^2 a_n implies a_{n+1} = dfrac{n+1}{n}a_n$$
First note that $a_0 = 0$. Choose $a_1$ arbitrarily. Then we get that $a_2 = 2a_1$, $a_3 = 3a_1$, $a_4 = 4a_1$ and in general, $a_{n} = na_1$.
Hence, the solution is given by
$$y(x) = a_1 left(x+2x^2 + 3x^3 + cdotsright)$$
This power series is valid only within $vert x vert <1$. In this region, we can simplify the power series to get
begin{align}
y(x) & = a_1 x left(1 + 2x + 3x^2 + cdots right)\
& = a_1 x dfrac{d}{dx} left(x + x^2 + x^3 + cdots right)\
& = a_1 x dfrac{d}{dx} left(dfrac{x}{1-x}right)\
& = a_1 dfrac{x}{(1-x)^2}
end{align}
$endgroup$
add a comment |
$begingroup$
The motivation behind Frobenius method is to seek a power series solution to ordinary differential equations.
Let $y(x) = displaystyle sum_{n=0}^{infty} a_n x^n$. Then we get that $$y'(x) = sum_{n=0}^{infty} na_n x^{n-1}$$
$$3xy'(x) = sum_{n=0}^{infty} 3na_n x^{n}$$
$$y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-2}$$
$$xy''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-1} = sum_{n=0}^{infty} n(n+1)a_{n+1} x^{n}$$
$$x^2y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n}$$
The ODE is $$xy'' - x^2 y'' -3xy' - y = 0$$
Plugging in the appropriate series expansions, we get that
$$sum_{n=0}^{infty} left(n(n+1)a_{n+1} - n(n-1)a_n - 3na_n - a_nright)x^n = 0$$
Hence, we get that
$$n(n+1)a_{n+1} = (n(n-1) +3n+1)a_n = (n+1)^2 a_n implies a_{n+1} = dfrac{n+1}{n}a_n$$
First note that $a_0 = 0$. Choose $a_1$ arbitrarily. Then we get that $a_2 = 2a_1$, $a_3 = 3a_1$, $a_4 = 4a_1$ and in general, $a_{n} = na_1$.
Hence, the solution is given by
$$y(x) = a_1 left(x+2x^2 + 3x^3 + cdotsright)$$
This power series is valid only within $vert x vert <1$. In this region, we can simplify the power series to get
begin{align}
y(x) & = a_1 x left(1 + 2x + 3x^2 + cdots right)\
& = a_1 x dfrac{d}{dx} left(x + x^2 + x^3 + cdots right)\
& = a_1 x dfrac{d}{dx} left(dfrac{x}{1-x}right)\
& = a_1 dfrac{x}{(1-x)^2}
end{align}
$endgroup$
The motivation behind Frobenius method is to seek a power series solution to ordinary differential equations.
Let $y(x) = displaystyle sum_{n=0}^{infty} a_n x^n$. Then we get that $$y'(x) = sum_{n=0}^{infty} na_n x^{n-1}$$
$$3xy'(x) = sum_{n=0}^{infty} 3na_n x^{n}$$
$$y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-2}$$
$$xy''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n-1} = sum_{n=0}^{infty} n(n+1)a_{n+1} x^{n}$$
$$x^2y''(x) = sum_{n=0}^{infty} n(n-1)a_n x^{n}$$
The ODE is $$xy'' - x^2 y'' -3xy' - y = 0$$
Plugging in the appropriate series expansions, we get that
$$sum_{n=0}^{infty} left(n(n+1)a_{n+1} - n(n-1)a_n - 3na_n - a_nright)x^n = 0$$
Hence, we get that
$$n(n+1)a_{n+1} = (n(n-1) +3n+1)a_n = (n+1)^2 a_n implies a_{n+1} = dfrac{n+1}{n}a_n$$
First note that $a_0 = 0$. Choose $a_1$ arbitrarily. Then we get that $a_2 = 2a_1$, $a_3 = 3a_1$, $a_4 = 4a_1$ and in general, $a_{n} = na_1$.
Hence, the solution is given by
$$y(x) = a_1 left(x+2x^2 + 3x^3 + cdotsright)$$
This power series is valid only within $vert x vert <1$. In this region, we can simplify the power series to get
begin{align}
y(x) & = a_1 x left(1 + 2x + 3x^2 + cdots right)\
& = a_1 x dfrac{d}{dx} left(x + x^2 + x^3 + cdots right)\
& = a_1 x dfrac{d}{dx} left(dfrac{x}{1-x}right)\
& = a_1 dfrac{x}{(1-x)^2}
end{align}
answered Oct 28 '12 at 5:23
user17762
add a comment |
add a comment |
$begingroup$
The order reduction method seeks a second basis solution in the form $y=y_1u$, where $y_1(x)=frac{x}{(1-x)^2}$ is the already found basis solution.
$$
x(1-x)[y_1u''+2y_1'u']-3x[y_1u']=0
implies frac{u''}{u'}=frac{3y_1-2(1-x)y_1'}{(1-x)y_1}
$$
Insert $y_1(x)=dfrac1{(1-x)^2}-dfrac1{1-x}$, $y_1'=dfrac2{(1-x)^3}-dfrac1{(1-x)^2}$, $y_1''=dfrac{6}{(1-x)^4}-dfrac{2}{(1-x)^3}$
into that formula to find
begin{align}
frac{u''}{u'}&=frac{-frac1{(1-x)^2}-frac1{1-x}}{frac{x}{1-x}}=-frac{2-x}{x(1-x)}=-frac{2}x+frac1{1-x}
\
implies u'&=frac1{x^2(1-x)}=frac{1+x}{x^2}-frac1{1-x}
\
implies u&=-frac1x+ln|x(1-x)|
end{align}
so that the second basis solution is
$$
y_2=frac{xln|x(1-x)|-1}{(1-x)^2}
$$
$endgroup$
add a comment |
$begingroup$
The order reduction method seeks a second basis solution in the form $y=y_1u$, where $y_1(x)=frac{x}{(1-x)^2}$ is the already found basis solution.
$$
x(1-x)[y_1u''+2y_1'u']-3x[y_1u']=0
implies frac{u''}{u'}=frac{3y_1-2(1-x)y_1'}{(1-x)y_1}
$$
Insert $y_1(x)=dfrac1{(1-x)^2}-dfrac1{1-x}$, $y_1'=dfrac2{(1-x)^3}-dfrac1{(1-x)^2}$, $y_1''=dfrac{6}{(1-x)^4}-dfrac{2}{(1-x)^3}$
into that formula to find
begin{align}
frac{u''}{u'}&=frac{-frac1{(1-x)^2}-frac1{1-x}}{frac{x}{1-x}}=-frac{2-x}{x(1-x)}=-frac{2}x+frac1{1-x}
\
implies u'&=frac1{x^2(1-x)}=frac{1+x}{x^2}-frac1{1-x}
\
implies u&=-frac1x+ln|x(1-x)|
end{align}
so that the second basis solution is
$$
y_2=frac{xln|x(1-x)|-1}{(1-x)^2}
$$
$endgroup$
add a comment |
$begingroup$
The order reduction method seeks a second basis solution in the form $y=y_1u$, where $y_1(x)=frac{x}{(1-x)^2}$ is the already found basis solution.
$$
x(1-x)[y_1u''+2y_1'u']-3x[y_1u']=0
implies frac{u''}{u'}=frac{3y_1-2(1-x)y_1'}{(1-x)y_1}
$$
Insert $y_1(x)=dfrac1{(1-x)^2}-dfrac1{1-x}$, $y_1'=dfrac2{(1-x)^3}-dfrac1{(1-x)^2}$, $y_1''=dfrac{6}{(1-x)^4}-dfrac{2}{(1-x)^3}$
into that formula to find
begin{align}
frac{u''}{u'}&=frac{-frac1{(1-x)^2}-frac1{1-x}}{frac{x}{1-x}}=-frac{2-x}{x(1-x)}=-frac{2}x+frac1{1-x}
\
implies u'&=frac1{x^2(1-x)}=frac{1+x}{x^2}-frac1{1-x}
\
implies u&=-frac1x+ln|x(1-x)|
end{align}
so that the second basis solution is
$$
y_2=frac{xln|x(1-x)|-1}{(1-x)^2}
$$
$endgroup$
The order reduction method seeks a second basis solution in the form $y=y_1u$, where $y_1(x)=frac{x}{(1-x)^2}$ is the already found basis solution.
$$
x(1-x)[y_1u''+2y_1'u']-3x[y_1u']=0
implies frac{u''}{u'}=frac{3y_1-2(1-x)y_1'}{(1-x)y_1}
$$
Insert $y_1(x)=dfrac1{(1-x)^2}-dfrac1{1-x}$, $y_1'=dfrac2{(1-x)^3}-dfrac1{(1-x)^2}$, $y_1''=dfrac{6}{(1-x)^4}-dfrac{2}{(1-x)^3}$
into that formula to find
begin{align}
frac{u''}{u'}&=frac{-frac1{(1-x)^2}-frac1{1-x}}{frac{x}{1-x}}=-frac{2-x}{x(1-x)}=-frac{2}x+frac1{1-x}
\
implies u'&=frac1{x^2(1-x)}=frac{1+x}{x^2}-frac1{1-x}
\
implies u&=-frac1x+ln|x(1-x)|
end{align}
so that the second basis solution is
$$
y_2=frac{xln|x(1-x)|-1}{(1-x)^2}
$$
answered Dec 20 '18 at 18:42
LutzLLutzL
59.8k42057
59.8k42057
add a comment |
add a comment |
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