Internal problem ID [1430]
Internal file name [OUTPUT/1431_Sunday_June_05_2022_02_16_59_AM_71911011/index.tex]
Book: Elementary differential equations with boundary value problems. William F. Trench.
Brooks/Cole 2001
Section: Chapter 7 Series Solutions of Linear Second Equations. 7.6 THE METHOD OF
FROBENIUS III. Exercises 7.7. Page 389
Problem number: 14.
ODE order: 2.
ODE degree: 1.
The type(s) of ODE detected by this program : "second order series method. Regular singular point. Difference is integer"
Maple gives the following as the ode type
[[_2nd_order, _with_linear_symmetries]]
\[ \boxed {x^{2} \left (2 x +1\right ) y^{\prime \prime }+x \left (9+13 x \right ) y^{\prime }+\left (7+5 x \right ) y=0} \] With the expansion point for the power series method at \(x = 0\).
The type of the expansion point is first determined. This is done on the homogeneous part of the ODE. \[ \left (2 x^{3}+x^{2}\right ) y^{\prime \prime }+\left (13 x^{2}+9 x \right ) y^{\prime }+\left (7+5 x \right ) y = 0 \] The following is summary of singularities for the above ode. Writing the ode as \begin {align*} y^{\prime \prime }+p(x) y^{\prime } + q(x) y &=0 \end {align*}
Where \begin {align*} p(x) &= \frac {9+13 x}{x \left (2 x +1\right )}\\ q(x) &= \frac {7+5 x}{x^{2} \left (2 x +1\right )}\\ \end {align*}
| \(p(x)=\frac {9+13 x}{x \left (2 x +1\right )}\) | |
| singularity | type |
| \(x = 0\) | \(\text {``regular''}\) |
| \(x = -{\frac {1}{2}}\) | \(\text {``regular''}\) |
| \(q(x)=\frac {7+5 x}{x^{2} \left (2 x +1\right )}\) | |
| singularity | type |
| \(x = 0\) | \(\text {``regular''}\) |
| \(x = -{\frac {1}{2}}\) | \(\text {``regular''}\) |
Combining everything together gives the following summary of singularities for the ode as
Regular singular points : \(\left [0, -{\frac {1}{2}}, \infty \right ]\)
Irregular singular points : \([]\)
Since \(x = 0\) is regular singular point, then Frobenius power series is used. The ode is normalized to be \[ x^{2} \left (2 x +1\right ) y^{\prime \prime }+\left (13 x^{2}+9 x \right ) y^{\prime }+\left (7+5 x \right ) y = 0 \] Let the solution be represented as Frobenius power series of the form \[ y = \moverset {\infty }{\munderset {n =0}{\sum }}a_{n} x^{n +r} \] Then \begin{align*} y^{\prime } &= \moverset {\infty }{\munderset {n =0}{\sum }}\left (n +r \right ) a_{n} x^{n +r -1} \\ y^{\prime \prime } &= \moverset {\infty }{\munderset {n =0}{\sum }}\left (n +r \right ) \left (n +r -1\right ) a_{n} x^{n +r -2} \\ \end{align*} Substituting the above back into the ode gives \begin{equation} \tag{1} x^{2} \left (2 x +1\right ) \left (\moverset {\infty }{\munderset {n =0}{\sum }}\left (n +r \right ) \left (n +r -1\right ) a_{n} x^{n +r -2}\right )+\left (13 x^{2}+9 x \right ) \left (\moverset {\infty }{\munderset {n =0}{\sum }}\left (n +r \right ) a_{n} x^{n +r -1}\right )+\left (7+5 x \right ) \left (\moverset {\infty }{\munderset {n =0}{\sum }}a_{n} x^{n +r}\right ) = 0 \end{equation} Which simplifies to \begin{equation} \tag{2A} \left (\moverset {\infty }{\munderset {n =0}{\sum }}2 x^{1+n +r} a_{n} \left (n +r \right ) \left (n +r -1\right )\right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}x^{n +r} a_{n} \left (n +r \right ) \left (n +r -1\right )\right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}13 x^{1+n +r} a_{n} \left (n +r \right )\right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}9 x^{n +r} a_{n} \left (n +r \right )\right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}7 a_{n} x^{n +r}\right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}5 x^{1+n +r} a_{n}\right ) = 0 \end{equation} The next step is to make all powers of \(x\) be \(n +r\) in each summation term. Going over each summation term above with power of \(x\) in it which is not already \(x^{n +r}\) and adjusting the power and the corresponding index gives \begin{align*} \moverset {\infty }{\munderset {n =0}{\sum }}2 x^{1+n +r} a_{n} \left (n +r \right ) \left (n +r -1\right ) &= \moverset {\infty }{\munderset {n =1}{\sum }}2 a_{n -1} \left (n +r -1\right ) \left (n +r -2\right ) x^{n +r} \\ \moverset {\infty }{\munderset {n =0}{\sum }}13 x^{1+n +r} a_{n} \left (n +r \right ) &= \moverset {\infty }{\munderset {n =1}{\sum }}13 a_{n -1} \left (n +r -1\right ) x^{n +r} \\ \moverset {\infty }{\munderset {n =0}{\sum }}5 x^{1+n +r} a_{n} &= \moverset {\infty }{\munderset {n =1}{\sum }}5 a_{n -1} x^{n +r} \\ \end{align*} Substituting all the above in Eq (2A) gives the following equation where now all powers of \(x\) are the same and equal to \(n +r\). \begin{equation} \tag{2B} \left (\moverset {\infty }{\munderset {n =1}{\sum }}2 a_{n -1} \left (n +r -1\right ) \left (n +r -2\right ) x^{n +r}\right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}x^{n +r} a_{n} \left (n +r \right ) \left (n +r -1\right )\right )+\left (\moverset {\infty }{\munderset {n =1}{\sum }}13 a_{n -1} \left (n +r -1\right ) x^{n +r}\right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}9 x^{n +r} a_{n} \left (n +r \right )\right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}7 a_{n} x^{n +r}\right )+\left (\moverset {\infty }{\munderset {n =1}{\sum }}5 a_{n -1} x^{n +r}\right ) = 0 \end{equation} The indicial equation is obtained from \(n = 0\). From Eq (2B) this gives \[ x^{n +r} a_{n} \left (n +r \right ) \left (n +r -1\right )+9 x^{n +r} a_{n} \left (n +r \right )+7 a_{n} x^{n +r} = 0 \] When \(n = 0\) the above becomes \[ x^{r} a_{0} r \left (-1+r \right )+9 x^{r} a_{0} r +7 a_{0} x^{r} = 0 \] Or \[ \left (x^{r} r \left (-1+r \right )+9 x^{r} r +7 x^{r}\right ) a_{0} = 0 \] Since \(a_{0}\neq 0\) then the above simplifies to \[ \left (r^{2}+8 r +7\right ) x^{r} = 0 \] Since the above is true for all \(x\) then the indicial equation becomes \[ r^{2}+8 r +7 = 0 \] Solving for \(r\) gives the roots of the indicial equation as \begin {align*} r_1 &= -1\\ r_2 &= -7 \end {align*}
Since \(a_{0}\neq 0\) then the indicial equation becomes \[ \left (r^{2}+8 r +7\right ) x^{r} = 0 \] Solving for \(r\) gives the roots of the indicial equation as \([-1, -7]\).
Since \(r_1 - r_2 = 6\) is an integer, then we can construct two linearly independent solutions \begin {align*} y_{1}\left (x \right ) &= x^{r_{1}} \left (\moverset {\infty }{\munderset {n =0}{\sum }}a_{n} x^{n}\right )\\ y_{2}\left (x \right ) &= C y_{1}\left (x \right ) \ln \left (x \right )+x^{r_{2}} \left (\moverset {\infty }{\munderset {n =0}{\sum }}b_{n} x^{n}\right ) \end {align*}
Or \begin {align*} y_{1}\left (x \right ) &= \frac {\moverset {\infty }{\munderset {n =0}{\sum }}a_{n} x^{n}}{x}\\ y_{2}\left (x \right ) &= C y_{1}\left (x \right ) \ln \left (x \right )+\frac {\moverset {\infty }{\munderset {n =0}{\sum }}b_{n} x^{n}}{x^{7}} \end {align*}
Or \begin {align*} y_{1}\left (x \right ) &= \moverset {\infty }{\munderset {n =0}{\sum }}a_{n} x^{n -1}\\ y_{2}\left (x \right ) &= C y_{1}\left (x \right ) \ln \left (x \right )+\left (\moverset {\infty }{\munderset {n =0}{\sum }}b_{n} x^{n -7}\right ) \end {align*}
Where \(C\) above can be zero. We start by finding \(y_{1}\). Eq (2B) derived above is now used to find all \(a_{n}\) coefficients. The case \(n = 0\) is skipped since it was used to find the roots of the indicial equation. \(a_{0}\) is arbitrary and taken as \(a_{0} = 1\). For \(1\le n\) the recursive equation is \begin{equation} \tag{3} 2 a_{n -1} \left (n +r -1\right ) \left (n +r -2\right )+a_{n} \left (n +r \right ) \left (n +r -1\right )+13 a_{n -1} \left (n +r -1\right )+9 a_{n} \left (n +r \right )+7 a_{n}+5 a_{n -1} = 0 \end{equation} Solving for \(a_{n}\) from recursive equation (4) gives \[ a_{n} = -\frac {a_{n -1} \left (2 n^{2}+4 n r +2 r^{2}+7 n +7 r -4\right )}{n^{2}+2 n r +r^{2}+8 n +8 r +7}\tag {4} \] Which for the root \(r = -1\) becomes \[ a_{n} = -\frac {a_{n -1} \left (2 n^{2}+3 n -9\right )}{n \left (n +6\right )}\tag {5} \] At this point, it is a good idea to keep track of \(a_{n}\) in a table both before substituting \(r = -1\) and after as more terms are found using the above recursive equation.
| \(n\) | \(a_{n ,r}\) | \(a_{n}\) |
| \(a_{0}\) | \(1\) | \(1\) |
For \(n = 1\), using the above recursive equation gives \[ a_{1}=\frac {-2 r^{2}-11 r -5}{r^{2}+10 r +16} \] Which for the root \(r = -1\) becomes \[ a_{1}={\frac {4}{7}} \] And the table now becomes
| \(n\) | \(a_{n ,r}\) | \(a_{n}\) |
| \(a_{0}\) | \(1\) | \(1\) |
| \(a_{1}\) | \(\frac {-2 r^{2}-11 r -5}{r^{2}+10 r +16}\) | \(\frac {4}{7}\) |
For \(n = 2\), using the above recursive equation gives \[ a_{2}=\frac {4 r^{4}+52 r^{3}+211 r^{2}+273 r +90}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right )} \] Which for the root \(r = -1\) becomes \[ a_{2}=-{\frac {5}{28}} \] And the table now becomes
| \(n\) | \(a_{n ,r}\) | \(a_{n}\) |
| \(a_{0}\) | \(1\) | \(1\) |
| \(a_{1}\) | \(\frac {-2 r^{2}-11 r -5}{r^{2}+10 r +16}\) | \(\frac {4}{7}\) |
| \(a_{2}\) | \(\frac {4 r^{4}+52 r^{3}+211 r^{2}+273 r +90}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right )}\) | \(-{\frac {5}{28}}\) |
For \(n = 3\), using the above recursive equation gives \[ a_{3}=\frac {-8 r^{6}-180 r^{5}-1550 r^{4}-6375 r^{3}-12752 r^{2}-11265 r -3150}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right ) \left (r +10\right ) \left (4+r \right )} \] Which for the root \(r = -1\) becomes \[ a_{3}={\frac {5}{42}} \] And the table now becomes
| \(n\) | \(a_{n ,r}\) | \(a_{n}\) |
| \(a_{0}\) | \(1\) | \(1\) |
| \(a_{1}\) | \(\frac {-2 r^{2}-11 r -5}{r^{2}+10 r +16}\) | \(\frac {4}{7}\) |
| \(a_{2}\) | \(\frac {4 r^{4}+52 r^{3}+211 r^{2}+273 r +90}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right )}\) | \(-{\frac {5}{28}}\) |
| \(a_{3}\) | \(\frac {-8 r^{6}-180 r^{5}-1550 r^{4}-6375 r^{3}-12752 r^{2}-11265 r -3150}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right ) \left (r +10\right ) \left (4+r \right )}\) | \(\frac {5}{42}\) |
For \(n = 4\), using the above recursive equation gives \[ a_{4}=\frac {16 r^{6}+336 r^{5}+2680 r^{4}+10200 r^{3}+19129 r^{2}+16149 r +4410}{\left (r +11\right ) \left (4+r \right ) \left (r +10\right ) \left (3+r \right ) \left (r +9\right ) \left (2+r \right )} \] Which for the root \(r = -1\) becomes \[ a_{4}=-{\frac {5}{48}} \] And the table now becomes
| \(n\) | \(a_{n ,r}\) | \(a_{n}\) |
| \(a_{0}\) | \(1\) | \(1\) |
| \(a_{1}\) | \(\frac {-2 r^{2}-11 r -5}{r^{2}+10 r +16}\) | \(\frac {4}{7}\) |
| \(a_{2}\) | \(\frac {4 r^{4}+52 r^{3}+211 r^{2}+273 r +90}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right )}\) | \(-{\frac {5}{28}}\) |
| \(a_{3}\) | \(\frac {-8 r^{6}-180 r^{5}-1550 r^{4}-6375 r^{3}-12752 r^{2}-11265 r -3150}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right ) \left (r +10\right ) \left (4+r \right )}\) | \(\frac {5}{42}\) |
| \(a_{4}\) | \(\frac {16 r^{6}+336 r^{5}+2680 r^{4}+10200 r^{3}+19129 r^{2}+16149 r +4410}{\left (r +11\right ) \left (4+r \right ) \left (r +10\right ) \left (3+r \right ) \left (r +9\right ) \left (2+r \right )}\) | \(-{\frac {5}{48}}\) |
For \(n = 5\), using the above recursive equation gives \[ a_{5}=\frac {-32 r^{6}-624 r^{5}-4640 r^{4}-16680 r^{3}-29978 r^{2}-24591 r -6615}{\left (r +12\right ) \left (2+r \right ) \left (3+r \right ) \left (r +10\right ) \left (4+r \right ) \left (r +11\right )} \] Which for the root \(r = -1\) becomes \[ a_{5}={\frac {7}{66}} \] And the table now becomes
| \(n\) | \(a_{n ,r}\) | \(a_{n}\) |
| \(a_{0}\) | \(1\) | \(1\) |
| \(a_{1}\) | \(\frac {-2 r^{2}-11 r -5}{r^{2}+10 r +16}\) | \(\frac {4}{7}\) |
| \(a_{2}\) | \(\frac {4 r^{4}+52 r^{3}+211 r^{2}+273 r +90}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right )}\) | \(-{\frac {5}{28}}\) |
| \(a_{3}\) | \(\frac {-8 r^{6}-180 r^{5}-1550 r^{4}-6375 r^{3}-12752 r^{2}-11265 r -3150}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right ) \left (r +10\right ) \left (4+r \right )}\) | \(\frac {5}{42}\) |
| \(a_{4}\) | \(\frac {16 r^{6}+336 r^{5}+2680 r^{4}+10200 r^{3}+19129 r^{2}+16149 r +4410}{\left (r +11\right ) \left (4+r \right ) \left (r +10\right ) \left (3+r \right ) \left (r +9\right ) \left (2+r \right )}\) | \(-{\frac {5}{48}}\) |
| \(a_{5}\) | \(\frac {-32 r^{6}-624 r^{5}-4640 r^{4}-16680 r^{3}-29978 r^{2}-24591 r -6615}{\left (r +12\right ) \left (2+r \right ) \left (3+r \right ) \left (r +10\right ) \left (4+r \right ) \left (r +11\right )}\) | \(\frac {7}{66}\) |
For \(n = 6\), using the above recursive equation gives \[ a_{6}=\frac {64 r^{6}+1152 r^{5}+8080 r^{4}+27840 r^{3}+48556 r^{2}+39048 r +10395}{\left (r +13\right ) \left (r +11\right ) \left (4+r \right ) \left (3+r \right ) \left (2+r \right ) \left (r +12\right )} \] Which for the root \(r = -1\) becomes \[ a_{6}=-{\frac {21}{176}} \] And the table now becomes
| \(n\) | \(a_{n ,r}\) | \(a_{n}\) |
| \(a_{0}\) | \(1\) | \(1\) |
| \(a_{1}\) | \(\frac {-2 r^{2}-11 r -5}{r^{2}+10 r +16}\) | \(\frac {4}{7}\) |
| \(a_{2}\) | \(\frac {4 r^{4}+52 r^{3}+211 r^{2}+273 r +90}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right )}\) | \(-{\frac {5}{28}}\) |
| \(a_{3}\) | \(\frac {-8 r^{6}-180 r^{5}-1550 r^{4}-6375 r^{3}-12752 r^{2}-11265 r -3150}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right ) \left (r +10\right ) \left (4+r \right )}\) | \(\frac {5}{42}\) |
| \(a_{4}\) | \(\frac {16 r^{6}+336 r^{5}+2680 r^{4}+10200 r^{3}+19129 r^{2}+16149 r +4410}{\left (r +11\right ) \left (4+r \right ) \left (r +10\right ) \left (3+r \right ) \left (r +9\right ) \left (2+r \right )}\) | \(-{\frac {5}{48}}\) |
| \(a_{5}\) | \(\frac {-32 r^{6}-624 r^{5}-4640 r^{4}-16680 r^{3}-29978 r^{2}-24591 r -6615}{\left (r +12\right ) \left (2+r \right ) \left (3+r \right ) \left (r +10\right ) \left (4+r \right ) \left (r +11\right )}\) | \(\frac {7}{66}\) |
| \(a_{6}\) | \(\frac {64 r^{6}+1152 r^{5}+8080 r^{4}+27840 r^{3}+48556 r^{2}+39048 r +10395}{\left (r +13\right ) \left (r +11\right ) \left (4+r \right ) \left (3+r \right ) \left (2+r \right ) \left (r +12\right )}\) | \(-{\frac {21}{176}}\) |
Using the above table, then the solution \(y_{1}\left (x \right )\) is \begin {align*} y_{1}\left (x \right )&= \frac {1}{x} \left (a_{0}+a_{1} x +a_{2} x^{2}+a_{3} x^{3}+a_{4} x^{4}+a_{5} x^{5}+a_{6} x^{6}+a_{7} x^{7}\dots \right ) \\ &= \frac {1+\frac {4 x}{7}-\frac {5 x^{2}}{28}+\frac {5 x^{3}}{42}-\frac {5 x^{4}}{48}+\frac {7 x^{5}}{66}-\frac {21 x^{6}}{176}+O\left (x^{7}\right )}{x} \end {align*}
Now the second solution \(y_{2}\left (x \right )\) is found. Let \[ r_{1}-r_{2} = N \] Where \(N\) is positive integer which is the difference between the two roots. \(r_{1}\) is taken as the larger root. Hence for this problem we have \(N=6\). Now we need to determine if \(C\) is zero or not. This is done by finding \(\lim _{r\rightarrow r_{2}}a_{6}\left (r \right )\). If this limit exists, then \(C = 0\), else we need to keep the log term and \(C \neq 0\). The above table shows that \begin {align*} a_N &= a_{6} \\ &= \frac {64 r^{6}+1152 r^{5}+8080 r^{4}+27840 r^{3}+48556 r^{2}+39048 r +10395}{\left (r +13\right ) \left (r +11\right ) \left (4+r \right ) \left (3+r \right ) \left (2+r \right ) \left (r +12\right )} \end {align*}
Therefore \begin {align*} \lim _{r\rightarrow r_{2}}\frac {64 r^{6}+1152 r^{5}+8080 r^{4}+27840 r^{3}+48556 r^{2}+39048 r +10395}{\left (r +13\right ) \left (r +11\right ) \left (4+r \right ) \left (3+r \right ) \left (2+r \right ) \left (r +12\right )}&= \lim _{r\rightarrow -7}\frac {64 r^{6}+1152 r^{5}+8080 r^{4}+27840 r^{3}+48556 r^{2}+39048 r +10395}{\left (r +13\right ) \left (r +11\right ) \left (4+r \right ) \left (3+r \right ) \left (2+r \right ) \left (r +12\right )}\\ &= -{\frac {3003}{160}} \end {align*}
The limit is \(-{\frac {3003}{160}}\). Since the limit exists then the log term is not needed and we can set \(C = 0\). Therefore the second solution has the form \begin {align*} y_{2}\left (x \right ) &= \moverset {\infty }{\munderset {n =0}{\sum }}b_{n} x^{n +r}\\ &= \moverset {\infty }{\munderset {n =0}{\sum }}b_{n} x^{n -7} \end {align*}
Eq (3) derived above is used to find all \(b_{n}\) coefficients. The case \(n = 0\) is skipped since it was used to find the roots of the indicial equation. \(b_{0}\) is arbitrary and taken as \(b_{0} = 1\). For \(1\le n\) the recursive equation is \begin{equation} \tag{4} 2 b_{n -1} \left (n +r -1\right ) \left (n +r -2\right )+b_{n} \left (n +r \right ) \left (n +r -1\right )+13 b_{n -1} \left (n +r -1\right )+9 b_{n} \left (n +r \right )+7 b_{n}+5 b_{n -1} = 0 \end{equation} Which for for the root \(r = -7\) becomes \begin{equation} \tag{4A} 2 b_{n -1} \left (n -8\right ) \left (n -9\right )+b_{n} \left (n -7\right ) \left (n -8\right )+13 b_{n -1} \left (n -8\right )+9 b_{n} \left (n -7\right )+7 b_{n}+5 b_{n -1} = 0 \end{equation} Solving for \(b_{n}\) from the recursive equation (4) gives \[ b_{n} = -\frac {b_{n -1} \left (2 n^{2}+4 n r +2 r^{2}+7 n +7 r -4\right )}{n^{2}+2 n r +r^{2}+8 n +8 r +7}\tag {5} \] Which for the root \(r = -7\) becomes \[ b_{n} = -\frac {b_{n -1} \left (2 n^{2}-21 n +45\right )}{n^{2}-6 n}\tag {6} \] At this point, it is a good idea to keep track of \(b_{n}\) in a table both before substituting \(r = -7\) and after as more terms are found using the above recursive equation.
| \(n\) | \(b_{n ,r}\) | \(b_{n}\) |
| \(b_{0}\) | \(1\) | \(1\) |
For \(n = 1\), using the above recursive equation gives \[ b_{1}=-\frac {2 r^{2}+11 r +5}{r^{2}+10 r +16} \] Which for the root \(r = -7\) becomes \[ b_{1}={\frac {26}{5}} \] And the table now becomes
| \(n\) | \(b_{n ,r}\) | \(b_{n}\) |
| \(b_{0}\) | \(1\) | \(1\) |
| \(b_{1}\) | \(\frac {-2 r^{2}-11 r -5}{r^{2}+10 r +16}\) | \(\frac {26}{5}\) |
For \(n = 2\), using the above recursive equation gives \[ b_{2}=\frac {4 r^{4}+52 r^{3}+211 r^{2}+273 r +90}{\left (r^{2}+10 r +16\right ) \left (r^{2}+12 r +27\right )} \] Which for the root \(r = -7\) becomes \[ b_{2}={\frac {143}{20}} \] And the table now becomes
| \(n\) | \(b_{n ,r}\) | \(b_{n}\) |
| \(b_{0}\) | \(1\) | \(1\) |
| \(b_{1}\) | \(\frac {-2 r^{2}-11 r -5}{r^{2}+10 r +16}\) | \(\frac {26}{5}\) |
| \(b_{2}\) | \(\frac {4 r^{4}+52 r^{3}+211 r^{2}+273 r +90}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right )}\) | \(\frac {143}{20}\) |
For \(n = 3\), using the above recursive equation gives \[ b_{3}=-\frac {8 r^{6}+180 r^{5}+1550 r^{4}+6375 r^{3}+12752 r^{2}+11265 r +3150}{\left (r^{2}+10 r +16\right ) \left (r^{2}+12 r +27\right ) \left (r^{2}+14 r +40\right )} \] Which for the root \(r = -7\) becomes \[ b_{3}=0 \] And the table now becomes
| \(n\) | \(b_{n ,r}\) | \(b_{n}\) |
| \(b_{0}\) | \(1\) | \(1\) |
| \(b_{1}\) | \(\frac {-2 r^{2}-11 r -5}{r^{2}+10 r +16}\) | \(\frac {26}{5}\) |
| \(b_{2}\) | \(\frac {4 r^{4}+52 r^{3}+211 r^{2}+273 r +90}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right )}\) | \(\frac {143}{20}\) |
| \(b_{3}\) | \(\frac {-8 r^{6}-180 r^{5}-1550 r^{4}-6375 r^{3}-12752 r^{2}-11265 r -3150}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right ) \left (r +10\right ) \left (4+r \right )}\) | \(0\) |
For \(n = 4\), using the above recursive equation gives \[ b_{4}=\frac {16 r^{6}+336 r^{5}+2680 r^{4}+10200 r^{3}+19129 r^{2}+16149 r +4410}{\left (r +11\right ) \left (r^{2}+14 r +40\right ) \left (r^{2}+12 r +27\right ) \left (2+r \right )} \] Which for the root \(r = -7\) becomes \[ b_{4}=0 \] And the table now becomes
| \(n\) | \(b_{n ,r}\) | \(b_{n}\) |
| \(b_{0}\) | \(1\) | \(1\) |
| \(b_{1}\) | \(\frac {-2 r^{2}-11 r -5}{r^{2}+10 r +16}\) | \(\frac {26}{5}\) |
| \(b_{2}\) | \(\frac {4 r^{4}+52 r^{3}+211 r^{2}+273 r +90}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right )}\) | \(\frac {143}{20}\) |
| \(b_{3}\) | \(\frac {-8 r^{6}-180 r^{5}-1550 r^{4}-6375 r^{3}-12752 r^{2}-11265 r -3150}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right ) \left (r +10\right ) \left (4+r \right )}\) | \(0\) |
| \(b_{4}\) | \(\frac {16 r^{6}+336 r^{5}+2680 r^{4}+10200 r^{3}+19129 r^{2}+16149 r +4410}{\left (r +11\right ) \left (4+r \right ) \left (r +10\right ) \left (3+r \right ) \left (r +9\right ) \left (2+r \right )}\) | \(0\) |
For \(n = 5\), using the above recursive equation gives \[ b_{5}=-\frac {32 r^{6}+624 r^{5}+4640 r^{4}+16680 r^{3}+29978 r^{2}+24591 r +6615}{\left (r +12\right ) \left (2+r \right ) \left (3+r \right ) \left (r^{2}+14 r +40\right ) \left (r +11\right )} \] Which for the root \(r = -7\) becomes \[ b_{5}=0 \] And the table now becomes
| \(n\) | \(b_{n ,r}\) | \(b_{n}\) |
| \(b_{0}\) | \(1\) | \(1\) |
| \(b_{1}\) | \(\frac {-2 r^{2}-11 r -5}{r^{2}+10 r +16}\) | \(\frac {26}{5}\) |
| \(b_{2}\) | \(\frac {4 r^{4}+52 r^{3}+211 r^{2}+273 r +90}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right )}\) | \(\frac {143}{20}\) |
| \(b_{3}\) | \(\frac {-8 r^{6}-180 r^{5}-1550 r^{4}-6375 r^{3}-12752 r^{2}-11265 r -3150}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right ) \left (r +10\right ) \left (4+r \right )}\) | \(0\) |
| \(b_{4}\) | \(\frac {16 r^{6}+336 r^{5}+2680 r^{4}+10200 r^{3}+19129 r^{2}+16149 r +4410}{\left (r +11\right ) \left (4+r \right ) \left (r +10\right ) \left (3+r \right ) \left (r +9\right ) \left (2+r \right )}\) | \(0\) |
| \(b_{5}\) | \(\frac {-32 r^{6}-624 r^{5}-4640 r^{4}-16680 r^{3}-29978 r^{2}-24591 r -6615}{\left (r +12\right ) \left (2+r \right ) \left (3+r \right ) \left (r +10\right ) \left (4+r \right ) \left (r +11\right )}\) | \(0\) |
For \(n = 6\), using the above recursive equation gives \[ b_{6}=\frac {64 r^{6}+1152 r^{5}+8080 r^{4}+27840 r^{3}+48556 r^{2}+39048 r +10395}{\left (r +13\right ) \left (r +11\right ) \left (4+r \right ) \left (3+r \right ) \left (2+r \right ) \left (r +12\right )} \] Which for the root \(r = -7\) becomes \[ b_{6}=-{\frac {3003}{160}} \] And the table now becomes
| \(n\) | \(b_{n ,r}\) | \(b_{n}\) |
| \(b_{0}\) | \(1\) | \(1\) |
| \(b_{1}\) | \(\frac {-2 r^{2}-11 r -5}{r^{2}+10 r +16}\) | \(\frac {26}{5}\) |
| \(b_{2}\) | \(\frac {4 r^{4}+52 r^{3}+211 r^{2}+273 r +90}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right )}\) | \(\frac {143}{20}\) |
| \(b_{3}\) | \(\frac {-8 r^{6}-180 r^{5}-1550 r^{4}-6375 r^{3}-12752 r^{2}-11265 r -3150}{\left (r +8\right ) \left (2+r \right ) \left (r +9\right ) \left (3+r \right ) \left (r +10\right ) \left (4+r \right )}\) | \(0\) |
| \(b_{4}\) | \(\frac {16 r^{6}+336 r^{5}+2680 r^{4}+10200 r^{3}+19129 r^{2}+16149 r +4410}{\left (r +11\right ) \left (4+r \right ) \left (r +10\right ) \left (3+r \right ) \left (r +9\right ) \left (2+r \right )}\) | \(0\) |
| \(b_{5}\) | \(\frac {-32 r^{6}-624 r^{5}-4640 r^{4}-16680 r^{3}-29978 r^{2}-24591 r -6615}{\left (r +12\right ) \left (2+r \right ) \left (3+r \right ) \left (r +10\right ) \left (4+r \right ) \left (r +11\right )}\) | \(0\) |
| \(b_{6}\) | \(\frac {64 r^{6}+1152 r^{5}+8080 r^{4}+27840 r^{3}+48556 r^{2}+39048 r +10395}{\left (r +13\right ) \left (r +11\right ) \left (4+r \right ) \left (3+r \right ) \left (2+r \right ) \left (r +12\right )}\) | \(-{\frac {3003}{160}}\) |
Using the above table, then the solution \(y_{2}\left (x \right )\) is \begin {align*} y_{2}\left (x \right )&= \frac {1}{x} \left (b_{0}+b_{1} x +b_{2} x^{2}+b_{3} x^{3}+b_{4} x^{4}+b_{5} x^{5}+b_{6} x^{6}+b_{7} x^{7}\dots \right ) \\ &= \frac {1+\frac {26 x}{5}+\frac {143 x^{2}}{20}-\frac {3003 x^{6}}{160}+O\left (x^{7}\right )}{x^{7}} \end {align*}
Therefore the homogeneous solution is \begin{align*} y_h(x) &= c_{1} y_{1}\left (x \right )+c_{2} y_{2}\left (x \right ) \\ &= \frac {c_{1} \left (1+\frac {4 x}{7}-\frac {5 x^{2}}{28}+\frac {5 x^{3}}{42}-\frac {5 x^{4}}{48}+\frac {7 x^{5}}{66}-\frac {21 x^{6}}{176}+O\left (x^{7}\right )\right )}{x} + \frac {c_{2} \left (1+\frac {26 x}{5}+\frac {143 x^{2}}{20}-\frac {3003 x^{6}}{160}+O\left (x^{7}\right )\right )}{x^{7}} \\ \end{align*} Hence the final solution is \begin{align*} y &= y_h \\ &= \frac {c_{1} \left (1+\frac {4 x}{7}-\frac {5 x^{2}}{28}+\frac {5 x^{3}}{42}-\frac {5 x^{4}}{48}+\frac {7 x^{5}}{66}-\frac {21 x^{6}}{176}+O\left (x^{7}\right )\right )}{x}+\frac {c_{2} \left (1+\frac {26 x}{5}+\frac {143 x^{2}}{20}-\frac {3003 x^{6}}{160}+O\left (x^{7}\right )\right )}{x^{7}} \\ \end{align*}
The solution(s) found are the following \begin{align*} \tag{1} y &= \frac {c_{1} \left (1+\frac {4 x}{7}-\frac {5 x^{2}}{28}+\frac {5 x^{3}}{42}-\frac {5 x^{4}}{48}+\frac {7 x^{5}}{66}-\frac {21 x^{6}}{176}+O\left (x^{7}\right )\right )}{x}+\frac {c_{2} \left (1+\frac {26 x}{5}+\frac {143 x^{2}}{20}-\frac {3003 x^{6}}{160}+O\left (x^{7}\right )\right )}{x^{7}} \\ \end{align*}
Verification of solutions
\[ y = \frac {c_{1} \left (1+\frac {4 x}{7}-\frac {5 x^{2}}{28}+\frac {5 x^{3}}{42}-\frac {5 x^{4}}{48}+\frac {7 x^{5}}{66}-\frac {21 x^{6}}{176}+O\left (x^{7}\right )\right )}{x}+\frac {c_{2} \left (1+\frac {26 x}{5}+\frac {143 x^{2}}{20}-\frac {3003 x^{6}}{160}+O\left (x^{7}\right )\right )}{x^{7}} \] Verified OK.
\[ \begin {array}{lll} & {} & \textrm {Let's solve}\hspace {3pt} \\ {} & {} & x^{2} \left (2 x +1\right ) \left (\frac {d}{d x}y^{\prime }\right )+\left (13 x^{2}+9 x \right ) y^{\prime }+\left (7+5 x \right ) y=0 \\ \bullet & {} & \textrm {Highest derivative means the order of the ODE is}\hspace {3pt} 2 \\ {} & {} & \frac {d}{d x}y^{\prime } \\ \bullet & {} & \textrm {Isolate 2nd derivative}\hspace {3pt} \\ {} & {} & \frac {d}{d x}y^{\prime }=-\frac {\left (7+5 x \right ) y}{x^{2} \left (2 x +1\right )}-\frac {\left (9+13 x \right ) y^{\prime }}{x \left (2 x +1\right )} \\ \bullet & {} & \textrm {Group terms with}\hspace {3pt} y\hspace {3pt}\textrm {on the lhs of the ODE and the rest on the rhs of the ODE; ODE is linear}\hspace {3pt} \\ {} & {} & \frac {d}{d x}y^{\prime }+\frac {\left (9+13 x \right ) y^{\prime }}{x \left (2 x +1\right )}+\frac {\left (7+5 x \right ) y}{x^{2} \left (2 x +1\right )}=0 \\ \square & {} & \textrm {Check to see if}\hspace {3pt} x_{0}\hspace {3pt}\textrm {is a regular singular point}\hspace {3pt} \\ {} & \circ & \textrm {Define functions}\hspace {3pt} \\ {} & {} & \left [P_{2}\left (x \right )=\frac {9+13 x}{x \left (2 x +1\right )}, P_{3}\left (x \right )=\frac {7+5 x}{x^{2} \left (2 x +1\right )}\right ] \\ {} & \circ & x \cdot P_{2}\left (x \right )\textrm {is analytic at}\hspace {3pt} x =0 \\ {} & {} & \left (x \cdot P_{2}\left (x \right )\right )\bigg | {\mstack {}{_{x \hiderel {=}0}}}=9 \\ {} & \circ & x^{2}\cdot P_{3}\left (x \right )\textrm {is analytic at}\hspace {3pt} x =0 \\ {} & {} & \left (x^{2}\cdot P_{3}\left (x \right )\right )\bigg | {\mstack {}{_{x \hiderel {=}0}}}=7 \\ {} & \circ & x =0\textrm {is a regular singular point}\hspace {3pt} \\ & {} & \textrm {Check to see if}\hspace {3pt} x_{0}\hspace {3pt}\textrm {is a regular singular point}\hspace {3pt} \\ {} & {} & x_{0}=0 \\ \bullet & {} & \textrm {Multiply by denominators}\hspace {3pt} \\ {} & {} & x^{2} \left (2 x +1\right ) \left (\frac {d}{d x}y^{\prime }\right )+x \left (9+13 x \right ) y^{\prime }+\left (7+5 x \right ) y=0 \\ \bullet & {} & \textrm {Assume series solution for}\hspace {3pt} y \\ {} & {} & y=\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} x^{k +r} \\ \square & {} & \textrm {Rewrite ODE with series expansions}\hspace {3pt} \\ {} & \circ & \textrm {Convert}\hspace {3pt} x^{m}\cdot y\hspace {3pt}\textrm {to series expansion for}\hspace {3pt} m =0..1 \\ {} & {} & x^{m}\cdot y=\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} x^{k +r +m} \\ {} & \circ & \textrm {Shift index using}\hspace {3pt} k \mathrm {->}k -m \\ {} & {} & x^{m}\cdot y=\moverset {\infty }{\munderset {k =m}{\sum }}a_{k -m} x^{k +r} \\ {} & \circ & \textrm {Convert}\hspace {3pt} x^{m}\cdot y^{\prime }\hspace {3pt}\textrm {to series expansion for}\hspace {3pt} m =1..2 \\ {} & {} & x^{m}\cdot y^{\prime }=\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} \left (k +r \right ) x^{k +r -1+m} \\ {} & \circ & \textrm {Shift index using}\hspace {3pt} k \mathrm {->}k +1-m \\ {} & {} & x^{m}\cdot y^{\prime }=\moverset {\infty }{\munderset {k =-1+m}{\sum }}a_{k +1-m} \left (k +1-m +r \right ) x^{k +r} \\ {} & \circ & \textrm {Convert}\hspace {3pt} x^{m}\cdot \left (\frac {d}{d x}y^{\prime }\right )\hspace {3pt}\textrm {to series expansion for}\hspace {3pt} m =2..3 \\ {} & {} & x^{m}\cdot \left (\frac {d}{d x}y^{\prime }\right )=\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} \left (k +r \right ) \left (k +r -1\right ) x^{k +r -2+m} \\ {} & \circ & \textrm {Shift index using}\hspace {3pt} k \mathrm {->}k +2-m \\ {} & {} & x^{m}\cdot \left (\frac {d}{d x}y^{\prime }\right )=\moverset {\infty }{\munderset {k =-2+m}{\sum }}a_{k +2-m} \left (k +2-m +r \right ) \left (k +1-m +r \right ) x^{k +r} \\ & {} & \textrm {Rewrite ODE with series expansions}\hspace {3pt} \\ {} & {} & a_{0} \left (7+r \right ) \left (1+r \right ) x^{r}+\left (\moverset {\infty }{\munderset {k =1}{\sum }}\left (a_{k} \left (k +r +7\right ) \left (k +r +1\right )+a_{k -1} \left (k +4+r \right ) \left (2 k -1+2 r \right )\right ) x^{k +r}\right )=0 \\ \bullet & {} & a_{0}\textrm {cannot be 0 by assumption, giving the indicial equation}\hspace {3pt} \\ {} & {} & \left (7+r \right ) \left (1+r \right )=0 \\ \bullet & {} & \textrm {Values of r that satisfy the indicial equation}\hspace {3pt} \\ {} & {} & r \in \left \{-7, -1\right \} \\ \bullet & {} & \textrm {Each term in the series must be 0, giving the recursion relation}\hspace {3pt} \\ {} & {} & 2 \left (k +4+r \right ) \left (k -\frac {1}{2}+r \right ) a_{k -1}+a_{k} \left (k +r +7\right ) \left (k +r +1\right )=0 \\ \bullet & {} & \textrm {Shift index using}\hspace {3pt} k \mathrm {->}k +1 \\ {} & {} & 2 \left (k +r +5\right ) \left (k +\frac {1}{2}+r \right ) a_{k}+a_{k +1} \left (k +8+r \right ) \left (k +2+r \right )=0 \\ \bullet & {} & \textrm {Recursion relation that defines series solution to ODE}\hspace {3pt} \\ {} & {} & a_{k +1}=-\frac {\left (k +r +5\right ) \left (2 k +2 r +1\right ) a_{k}}{\left (k +8+r \right ) \left (k +2+r \right )} \\ \bullet & {} & \textrm {Recursion relation for}\hspace {3pt} r =-7\hspace {3pt}\textrm {; series terminates at}\hspace {3pt} k =2 \\ {} & {} & a_{k +1}=-\frac {\left (k -2\right ) \left (2 k -13\right ) a_{k}}{\left (k +1\right ) \left (k -5\right )} \\ \bullet & {} & \textrm {Apply recursion relation for}\hspace {3pt} k =0 \\ {} & {} & a_{1}=\frac {26 a_{0}}{5} \\ \bullet & {} & \textrm {Apply recursion relation for}\hspace {3pt} k =1 \\ {} & {} & a_{2}=\frac {11 a_{1}}{8} \\ \bullet & {} & \textrm {Express in terms of}\hspace {3pt} a_{0} \\ {} & {} & a_{2}=\frac {143 a_{0}}{20} \\ \bullet & {} & \textrm {Terminating series solution of the ODE for}\hspace {3pt} r =-7\hspace {3pt}\textrm {. Use reduction of order to find the second linearly independent solution}\hspace {3pt} \\ {} & {} & y=a_{0}\cdot \left (1+\frac {26}{5} x +\frac {143}{20} x^{2}\right ) \\ \bullet & {} & \textrm {Recursion relation for}\hspace {3pt} r =-1 \\ {} & {} & a_{k +1}=-\frac {\left (k +4\right ) \left (2 k -1\right ) a_{k}}{\left (k +7\right ) \left (k +1\right )} \\ \bullet & {} & \textrm {Solution for}\hspace {3pt} r =-1 \\ {} & {} & \left [y=\moverset {\infty }{\munderset {k =0}{\sum }}a_{k} x^{k -1}, a_{k +1}=-\frac {\left (k +4\right ) \left (2 k -1\right ) a_{k}}{\left (k +7\right ) \left (k +1\right )}\right ] \\ \bullet & {} & \textrm {Combine solutions and rename parameters}\hspace {3pt} \\ {} & {} & \left [y=a_{0}\cdot \left (1+\frac {26}{5} x +\frac {143}{20} x^{2}\right )+\left (\moverset {\infty }{\munderset {k =0}{\sum }}b_{k} x^{k -1}\right ), b_{k +1}=-\frac {\left (k +4\right ) \left (2 k -1\right ) b_{k}}{\left (k +7\right ) \left (k +1\right )}\right ] \end {array} \]
Maple trace Kovacic algorithm successful
`Methods for second order ODEs: --- Trying classification methods --- trying a quadrature checking if the LODE has constant coefficients checking if the LODE is of Euler type trying a symmetry of the form [xi=0, eta=F(x)] checking if the LODE is missing y -> Trying a Liouvillian solution using Kovacics algorithm A Liouvillian solution exists Reducible group (found an exponential solution) Reducible group (found another exponential solution) <- Kovacics algorithm successful`
✓ Solution by Maple
Time used: 0.016 (sec). Leaf size: 41
Order:=6; dsolve(x^2*(1+2*x)*diff(y(x),x$2)+x*(9+13*x)*diff(y(x),x)+(7+5*x)*y(x)=0,y(x),type='series',x=0);
\[ y \left (x \right ) = \frac {c_{1} \left (1+\frac {4}{7} x -\frac {5}{28} x^{2}+\frac {5}{42} x^{3}-\frac {5}{48} x^{4}+\frac {7}{66} x^{5}+\operatorname {O}\left (x^{6}\right )\right )}{x}+\frac {c_{2} \left (-86400-449280 x -617760 x^{2}+\operatorname {O}\left (x^{6}\right )\right )}{x^{7}} \]
✓ Solution by Mathematica
Time used: 0.059 (sec). Leaf size: 54
AsymptoticDSolveValue[x^2*(1+2*x)*y''[x]+x*(9+13*x)*y'[x]+(7+5*x)*y[x]==0,y[x],{x,0,5}]
\[ y(x)\to c_2 \left (-\frac {5 x^3}{48}+\frac {5 x^2}{42}-\frac {5 x}{28}+\frac {1}{x}+\frac {4}{7}\right )+c_1 \left (\frac {1}{x^7}+\frac {26}{5 x^6}+\frac {143}{20 x^5}\right ) \]