Internal problem ID [10049]
Internal file name [OUTPUT/8996_Monday_June_06_2022_06_09_55_AM_96660773/index.tex]
Book: Differential Gleichungen, E. Kamke, 3rd ed. Chelsea Pub. NY, 1948
Section: Chapter 6, non-linear second order
Problem number: 1728 (book 6.137).
ODE order: 2.
ODE degree: 1.
The type(s) of ODE detected by this program : "second_order_ode_missing_x"
Maple gives the following as the ode type
[[_2nd_order, _missing_x], [_2nd_order, _reducible, _mu_x_y1]]
\[ \boxed {2 y^{\prime \prime } y+{y^{\prime }}^{2}=-1} \]
This is missing independent variable second order ode. Solved by reduction of order by using substitution which makes the dependent variable \(y\) an independent variable. Using \begin {align*} y' &= p(y) \end {align*}
Then \begin {align*} y'' &= \frac {dp}{dx}\\ &= \frac {dy}{dx} \frac {dp}{dy}\\ &= p \frac {dp}{dy} \end {align*}
Hence the ode becomes \begin {align*} 2 p \left (y \right ) \left (\frac {d}{d y}p \left (y \right )\right ) y +p \left (y \right )^{2} = -1 \end {align*}
Which is now solved as first order ode for \(p(y)\). In canonical form the ODE is \begin {align*} p' &= F(y,p)\\ &= f( y) g(p)\\ &= -\frac {p^{2}+1}{2 p y} \end {align*}
Where \(f(y)=-\frac {1}{2 y}\) and \(g(p)=\frac {p^{2}+1}{p}\). Integrating both sides gives \begin{align*} \frac {1}{\frac {p^{2}+1}{p}} \,dp &= -\frac {1}{2 y} \,d y \\ \int { \frac {1}{\frac {p^{2}+1}{p}} \,dp} &= \int {-\frac {1}{2 y} \,d y} \\ \frac {\ln \left (p^{2}+1\right )}{2}&=-\frac {\ln \left (y \right )}{2}+c_{1} \\ \end{align*} Raising both side to exponential gives \begin {align*} \sqrt {p^{2}+1} &= {\mathrm e}^{-\frac {\ln \left (y \right )}{2}+c_{1}} \end {align*}
Which simplifies to \begin {align*} \sqrt {p^{2}+1} &= \frac {c_{2}}{\sqrt {y}} \end {align*}
Which simplifies to \[ \sqrt {p \left (y \right )^{2}+1} = \frac {c_{2} {\mathrm e}^{c_{1}}}{\sqrt {y}} \] The solution is \[ \sqrt {p \left (y \right )^{2}+1} = \frac {c_{2} {\mathrm e}^{c_{1}}}{\sqrt {y}} \] For solution (1) found earlier, since \(p=y^{\prime }\) then we now have a new first order ode to solve which is \begin {align*} \sqrt {{y^{\prime }}^{2}+1} = \frac {c_{2} {\mathrm e}^{c_{1}}}{\sqrt {y}} \end {align*}
Solving the given ode for \(y^{\prime }\) results in \(2\) differential equations to solve. Each one of these will generate a solution. The equations generated are \begin {align*} y^{\prime }&=\frac {\sqrt {-y \left (-c_{2}^{2} {\mathrm e}^{2 c_{1}}+y\right )}}{y} \tag {1} \\ y^{\prime }&=-\frac {\sqrt {-y \left (-c_{2}^{2} {\mathrm e}^{2 c_{1}}+y\right )}}{y} \tag {2} \end {align*}
Now each one of the above ODE is solved.
Solving equation (1)
Integrating both sides gives \begin{align*} \int \frac {y}{\sqrt {-y \left (-c_{2}^{2} {\mathrm e}^{2 c_{1}}+y \right )}}d y &= \int d x \\ -\sqrt {y \,{\mathrm e}^{2 c_{1}} c_{2}^{2}-y^{2}}+\frac {c_{2}^{2} {\mathrm e}^{2 c_{1}} \arctan \left (\frac {y-\frac {c_{2}^{2} {\mathrm e}^{2 c_{1}}}{2}}{\sqrt {y \,{\mathrm e}^{2 c_{1}} c_{2}^{2}-y^{2}}}\right )}{2}&=x +c_{3} \\ \end{align*} Solving equation (2)
Integrating both sides gives \begin{align*} \int -\frac {y}{\sqrt {-y \left (-c_{2}^{2} {\mathrm e}^{2 c_{1}}+y \right )}}d y &= \int d x \\ \sqrt {y \,{\mathrm e}^{2 c_{1}} c_{2}^{2}-y^{2}}-\frac {c_{2}^{2} {\mathrm e}^{2 c_{1}} \arctan \left (\frac {y-\frac {c_{2}^{2} {\mathrm e}^{2 c_{1}}}{2}}{\sqrt {y \,{\mathrm e}^{2 c_{1}} c_{2}^{2}-y^{2}}}\right )}{2}&=x +c_{4} \\ \end{align*}
The solution(s) found are the following \begin{align*} \tag{1} -\sqrt {y \,{\mathrm e}^{2 c_{1}} c_{2}^{2}-y^{2}}+\frac {c_{2}^{2} {\mathrm e}^{2 c_{1}} \arctan \left (\frac {y-\frac {c_{2}^{2} {\mathrm e}^{2 c_{1}}}{2}}{\sqrt {y \,{\mathrm e}^{2 c_{1}} c_{2}^{2}-y^{2}}}\right )}{2} &= x +c_{3} \\ \tag{2} \sqrt {y \,{\mathrm e}^{2 c_{1}} c_{2}^{2}-y^{2}}-\frac {c_{2}^{2} {\mathrm e}^{2 c_{1}} \arctan \left (\frac {y-\frac {c_{2}^{2} {\mathrm e}^{2 c_{1}}}{2}}{\sqrt {y \,{\mathrm e}^{2 c_{1}} c_{2}^{2}-y^{2}}}\right )}{2} &= x +c_{4} \\ \end{align*}
Verification of solutions
\[ -\sqrt {y \,{\mathrm e}^{2 c_{1}} c_{2}^{2}-y^{2}}+\frac {c_{2}^{2} {\mathrm e}^{2 c_{1}} \arctan \left (\frac {y-\frac {c_{2}^{2} {\mathrm e}^{2 c_{1}}}{2}}{\sqrt {y \,{\mathrm e}^{2 c_{1}} c_{2}^{2}-y^{2}}}\right )}{2} = x +c_{3} \] Verified OK.
\[ \sqrt {y \,{\mathrm e}^{2 c_{1}} c_{2}^{2}-y^{2}}-\frac {c_{2}^{2} {\mathrm e}^{2 c_{1}} \arctan \left (\frac {y-\frac {c_{2}^{2} {\mathrm e}^{2 c_{1}}}{2}}{\sqrt {y \,{\mathrm e}^{2 c_{1}} c_{2}^{2}-y^{2}}}\right )}{2} = x +c_{4} \] Verified OK.
Maple trace
`Methods for second order ODEs: --- Trying classification methods --- trying 2nd order Liouville trying 2nd order WeierstrassP trying 2nd order JacobiSN differential order: 2; trying a linearization to 3rd order trying 2nd order ODE linearizable_by_differentiation trying 2nd order, 2 integrating factors of the form mu(x,y) trying differential order: 2; missing variables `, `-> Computing symmetries using: way = 3 -> Calling odsolve with the ODE`, (diff(_b(_a), _a))*_b(_a)+(1/2)*(_b(_a)^2+1)/_a = 0, _b(_a), HINT = [[_a, 0]]` *** Sublevel 2 ** symmetry methods on request `, `1st order, trying reduction of order with given symmetries:`[_a, 0]
✓ Solution by Maple
Time used: 0.062 (sec). Leaf size: 327
dsolve(2*diff(diff(y(x),x),x)*y(x)+diff(y(x),x)^2+1=0,y(x), singsol=all)
\begin{align*} y \left (x \right ) &= \frac {\left (-\operatorname {RootOf}\left (\left (-\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} -2 c_{2} -2 x \right ) \left (\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} -2 c_{2} -2 x \right )\right ) c_{1} +2 x +2 c_{2} \right ) \tan \left (\operatorname {RootOf}\left (\left (-\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} -2 c_{2} -2 x \right ) \left (\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} -2 c_{2} -2 x \right )\right )\right )}{2}+\frac {c_{1}}{2} \\ y \left (x \right ) &= \frac {\left (-\operatorname {RootOf}\left (\left (\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} +2 c_{2} +2 x \right ) \left (-\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} +2 c_{2} +2 x \right )\right ) c_{1} -2 x -2 c_{2} \right ) \tan \left (\operatorname {RootOf}\left (\left (\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} +2 c_{2} +2 x \right ) \left (-\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} +2 c_{2} +2 x \right )\right )\right )}{2}+\frac {c_{1}}{2} \\ y \left (x \right ) &= \frac {\left (\operatorname {RootOf}\left (\left (-\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} -2 c_{2} -2 x \right ) \left (\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} -2 c_{2} -2 x \right )\right ) c_{1} -2 x -2 c_{2} \right ) \tan \left (\operatorname {RootOf}\left (\left (-\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} -2 c_{2} -2 x \right ) \left (\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} -2 c_{2} -2 x \right )\right )\right )}{2}+\frac {c_{1}}{2} \\ y \left (x \right ) &= \frac {\left (\operatorname {RootOf}\left (\left (\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} +2 c_{2} +2 x \right ) \left (-\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} +2 c_{2} +2 x \right )\right ) c_{1} +2 x +2 c_{2} \right ) \tan \left (\operatorname {RootOf}\left (\left (\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} +2 c_{2} +2 x \right ) \left (-\cos \left (\textit {\_Z} \right ) c_{1} +c_{1} \textit {\_Z} +2 c_{2} +2 x \right )\right )\right )}{2}+\frac {c_{1}}{2} \\ \end{align*}
✓ Solution by Mathematica
Time used: 1.631 (sec). Leaf size: 397
DSolve[1 + y'[x]^2 + 2*y[x]*y''[x] == 0,y[x],x,IncludeSingularSolutions -> True]
\begin{align*} y(x)\to \text {InverseFunction}\left [-e^{2 c_1} \arctan \left (\frac {\sqrt {-\text {$\#$1}+e^{2 c_1}}}{\sqrt {\text {$\#$1}}}\right )-\sqrt {\text {$\#$1}} \sqrt {-\text {$\#$1}+e^{2 c_1}}\&\right ][x+c_2] \\ y(x)\to \text {InverseFunction}\left [e^{2 c_1} \arctan \left (\frac {\sqrt {-\text {$\#$1}+e^{2 c_1}}}{\sqrt {\text {$\#$1}}}\right )+\sqrt {\text {$\#$1}} \sqrt {-\text {$\#$1}+e^{2 c_1}}\&\right ][x+c_2] \\ y(x)\to \text {InverseFunction}\left [-e^{2 (-c_1)} \arctan \left (\frac {\sqrt {-\text {$\#$1}+e^{2 (-c_1)}}}{\sqrt {\text {$\#$1}}}\right )-\sqrt {\text {$\#$1}} \sqrt {-\text {$\#$1}+e^{2 (-c_1)}}\&\right ][x+c_2] \\ y(x)\to \text {InverseFunction}\left [e^{2 (-c_1)} \arctan \left (\frac {\sqrt {-\text {$\#$1}+e^{2 (-c_1)}}}{\sqrt {\text {$\#$1}}}\right )+\sqrt {\text {$\#$1}} \sqrt {-\text {$\#$1}+e^{2 (-c_1)}}\&\right ][x+c_2] \\ y(x)\to \text {InverseFunction}\left [-e^{2 c_1} \arctan \left (\frac {\sqrt {-\text {$\#$1}+e^{2 c_1}}}{\sqrt {\text {$\#$1}}}\right )-\sqrt {\text {$\#$1}} \sqrt {-\text {$\#$1}+e^{2 c_1}}\&\right ][x+c_2] \\ y(x)\to \text {InverseFunction}\left [e^{2 c_1} \arctan \left (\frac {\sqrt {-\text {$\#$1}+e^{2 c_1}}}{\sqrt {\text {$\#$1}}}\right )+\sqrt {\text {$\#$1}} \sqrt {-\text {$\#$1}+e^{2 c_1}}\&\right ][x+c_2] \\ \end{align*}