Internal problem ID [14708]
Internal file name [OUTPUT/14388_Wednesday_April_03_2024_02_17_58_PM_45871068/index.tex]
Book: INTRODUCTORY DIFFERENTIAL EQUATIONS. Martha L. Abell, James P. Braselton.
Fourth edition 2014. ElScAe. 2014
Section: Chapter 4. Higher Order Equations. Exercises 4.6, page 187
Problem number: 34.
ODE order: 4.
ODE degree: 1.
The type(s) of ODE detected by this program : "unknown"
Maple gives the following as the ode type
[[_high_order, _missing_y]]
Unable to solve or complete the solution.
\[ \boxed {t y^{\prime \prime \prime \prime }+2 y^{\prime \prime \prime }=\frac {45}{8 t^{\frac {7}{2}}}} \] With initial conditions \begin {align*} [y \left (1\right ) = 0, y^{\prime }\left (1\right ) = 0, y^{\prime \prime }\left (1\right ) = 1, y^{\prime \prime \prime }\left (1\right ) = 0] \end {align*}
Since \(y\) is missing from the ode then we can use the substitution \(y^{\prime } = v \left (t \right )\) to reduce the order by one. The ODE becomes \begin {align*} t v^{\prime \prime \prime }\left (t \right )+2 v^{\prime \prime }\left (t \right ) = 0 \end {align*}
Since \(v \left (t \right )\) is missing from the ode then we can use the substitution \(v^{\prime }\left (t \right ) = w \left (t \right )\) to reduce the order by one. The ODE becomes \begin {align*} t w^{\prime \prime }\left (t \right )+2 w^{\prime }\left (t \right ) = 0 \end {align*}
Integrating both sides of the ODE w.r.t \(t\) gives \begin {align*} \int \left (t w^{\prime \prime }\left (t \right )+2 w^{\prime }\left (t \right )\right )d t &= 0 \\ w^{\prime }\left (t \right ) t +w \left (t \right ) = c_{1} \end {align*}
Which is now solved for \(w \left (t \right )\). In canonical form the ODE is \begin {align*} w' &= F(t,w)\\ &= f( t) g(w)\\ &= \frac {-w +c_{1}}{t} \end {align*}
Where \(f(t)=\frac {1}{t}\) and \(g(w)=-w +c_{1}\). Integrating both sides gives \begin{align*} \frac {1}{-w +c_{1}} \,dw &= \frac {1}{t} \,d t \\ \int { \frac {1}{-w +c_{1}} \,dw} &= \int {\frac {1}{t} \,d t} \\ -\ln \left (w -c_{1} \right )&=\ln \left (t \right )+c_{2} \\ \end{align*} Raising both side to exponential gives \begin {align*} \frac {1}{w -c_{1}} &= {\mathrm e}^{\ln \left (t \right )+c_{2}} \end {align*}
Which simplifies to \begin {align*} \frac {1}{w -c_{1}} &= c_{3} t \end {align*}
Which simplifies to \[ w \left (t \right ) = \frac {\left (c_{3} {\mathrm e}^{c_{2}} t c_{1} +1\right ) {\mathrm e}^{-c_{2}}}{c_{3} t} \] But since \(v^{\prime }\left (t \right ) = w \left (t \right )\) then we now need to solve the ode \(v^{\prime }\left (t \right ) = \frac {\left (c_{3} {\mathrm e}^{c_{2}} t c_{1} +1\right ) {\mathrm e}^{-c_{2}}}{c_{3} t}\). Integrating both sides gives \begin {align*} v \left (t \right ) &= \int { \frac {\left (c_{3} {\mathrm e}^{c_{2}} t c_{1} +1\right ) {\mathrm e}^{-c_{2}}}{c_{3} t}\,\mathop {\mathrm {d}t}}\\ &= c_{1} t +\frac {{\mathrm e}^{-c_{2}} \ln \left (t \right )}{c_{3}}+c_{4} \end {align*}
But since \(y^{\prime } = v \left (t \right )\) then we now need to solve the ode \(y^{\prime } = c_{1} t +\frac {{\mathrm e}^{-c_{2}} \ln \left (t \right )}{c_{3}}+c_{4}\). Integrating both sides gives \begin {align*} y &= \int { \frac {c_{1} t c_{3} +{\mathrm e}^{-c_{2}} \ln \left (t \right )+c_{4} c_{3}}{c_{3}}\,\mathop {\mathrm {d}t}}\\ &= t c_{4} +\frac {t^{2} c_{1}}{2}+\frac {{\mathrm e}^{-c_{2}} \left (t \ln \left (t \right )-t \right )}{c_{3}}+c_{5} \end {align*}
This is higher order nonhomogeneous ODE. Let the solution be \[ y = y_h + y_p \] Where \(y_h\) is the solution to the homogeneous ODE And \(y_p\) is a particular solution to the nonhomogeneous ODE. \(y_h\) is the solution to \[ t y^{\prime \prime \prime \prime }+2 y^{\prime \prime \prime } = 0 \] Let the particular solution be \[ y_p = U_1 y_1+U_2 y_2+U_3 y_3+U_4 y_4 \] Where \(y_i\) are the basis solutions found above for the homogeneous solution \(y_h\) and \(U_i(t)\) are functions to be determined as follows \[ U_i = (-1)^{n-i} \int { \frac {F(t) W_i(t) }{a W(t)} \, dt} \] Where \(W(t)\) is the Wronskian and \(W_i(t)\) is the Wronskian that results after deleting the last row and the \(i\)-th column of the determinant and \(n\) is the order of the ODE or equivalently, the number of basis solutions, and \(a\) is the coefficient of the leading derivative in the ODE, and \(F(t)\) is the RHS of the ODE. Therefore, the first step is to find the Wronskian \(W \left (t \right )\). This is given by \begin {equation*} W(t) = \begin {vmatrix} y_1&y_2&y_3&y_4\\ y_1'&y_2'&y_3'&y_4'\\ y_1''&y_2''&y_3''&y_4''\\ y_1'''&y_2'''&y_3'''&y_4'''\\ \end {vmatrix} \end {equation*} Substituting the fundamental set of solutions \(y_i\) found above in the Wronskian gives \begin {align*} W &= \left [\begin {array}{ccc} 1 & t & t^{2} \\ 0 & 1 & 2 t \\ 0 & 0 & 2 \end {array}\right ] \\ |W| &= 2 \end {align*}
The determinant simplifies to \begin {align*} |W| &= 2 \end {align*}
Now we determine \(W_i\) for each \(U_i\). \begin {align*} W_1(t) &= \det \,\left [\begin {array}{cc} t & t^{2} \\ 1 & 2 t \end {array}\right ] \\ &= t^{2} \end {align*}
\begin {align*} W_2(t) &= \det \,\left [\begin {array}{cc} 1 & t^{2} \\ 0 & 2 t \end {array}\right ] \\ &= 2 t \end {align*}
\begin {align*} W_3(t) &= \det \,\left [\begin {array}{cc} 1 & t \\ 0 & 1 \end {array}\right ] \\ &= 1 \end {align*}
Unable to find particular solution. Terminating. Initial conditions are used to solve for the constants of integration.
\[ \begin {array}{lll} & {} & \textrm {Let's solve}\hspace {3pt} \\ {} & {} & \left [t \left (\frac {d}{d t}\frac {d}{d t}y^{\prime \prime }\right )+2 \frac {d}{d t}y^{\prime \prime }=\frac {45}{8 t^{\frac {7}{2}}}, y \left (1\right )=0, y^{\prime }{\raise{-0.36em}{\Big |}}{\mstack {}{_{\left \{t \hiderel {=}1\right \}}}}=0, \left (\frac {d}{d t}y^{\prime }\right )\bigg | {\mstack {}{_{\left \{t \hiderel {=}1\right \}}}}=1, \left (\frac {d}{d t}y^{\prime \prime }\right )\bigg | {\mstack {}{_{\left \{t \hiderel {=}1\right \}}}}=0\right ] \\ \bullet & {} & \textrm {Highest derivative means the order of the ODE is}\hspace {3pt} 4 \\ {} & {} & \frac {d}{d t}\frac {d}{d t}y^{\prime \prime } \end {array} \]
Maple trace
`Methods for high order ODEs: --- Trying classification methods --- trying a quadrature trying high order exact linear fully integrable -> Calling odsolve with the ODE`, diff(_b(_a), _a) = -(1/8)*(16*_b(_a)*_a^(7/2)-45)/_a^(9/2), _b(_a)` *** Sublevel 2 *** Methods for first order ODEs: --- Trying classification methods --- trying a quadrature trying 1st order linear <- 1st order linear successful <- high order exact linear fully integrable successful`
✓ Solution by Maple
Time used: 0.047 (sec). Leaf size: 24
dsolve([t*diff(y(t),t$4)+2*diff(y(t),t$3)=45/8*1/t^(7/2),y(1) = 0, D(y)(1) = 0, (D@@2)(y)(1) = 1, (D@@3)(y)(1) = 0],y(t), singsol=all)
\[ y \left (t \right ) = \frac {13 t^{2}}{8}+\frac {2}{\sqrt {t}}-\frac {15 t \ln \left (t \right )}{4}+\frac {3 t}{2}-\frac {41}{8} \]
✓ Solution by Mathematica
Time used: 0.098 (sec). Leaf size: 31
DSolve[{t*y''''[t]+2*y'''[t]==45/8*1/t^(7/2),{y[1]==0,y'[1]==0,y''[1]==1,y'''[1]==0}},y[t],t,IncludeSingularSolutions -> True]
\[ y(t)\to \frac {1}{8} \left (13 t^2+12 t+\frac {16}{\sqrt {t}}-30 t \log (t)-41\right ) \]