35.7.8 problem 5

Internal problem ID [6190]
Book : Mathematical Methods in the Physical Sciences. third edition. Mary L. Boas. John Wiley. 2006
Section : Chapter 8, Ordinary differential equations. Section 7. Other second-Order equations. page 435
Problem number : 5
Date solved : Wednesday, March 05, 2025 at 12:24:04 AM
CAS classification : [[_2nd_order, _missing_x]]

\begin{align*} {y^{\prime \prime }}^{2}&=k^{2} \left (1+{y^{\prime }}^{2}\right ) \end{align*}

✓ Maple. Time used: 0.506 (sec). Leaf size: 53

ode:=diff(diff(y(x),x),x)^2 = k^2*(1+diff(y(x),x)^2); 
dsolve(ode,y(x), singsol=all);
 

\begin{align*} y &= -i x +c_{1} \\ y &= i x +c_{1} \\ y &= \frac {4 c_{2}^{2} {\mathrm e}^{k x} k^{2}+4 c_{1} c_{2} k^{2}+{\mathrm e}^{-k x}}{4 c_{2} k^{2}} \\ \end{align*}

✓ Mathematica. Time used: 0.524 (sec). Leaf size: 40

ode=(D[y[x],{x,2}])^2==k^2*(1+ (D[y[x],x])^2); 
ic={}; 
DSolve[{ode,ic},y[x],x,IncludeSingularSolutions->True]
 

\begin{align*} y(x)\to c_2-\frac {\cosh (k x-c_1)}{k} \\ y(x)\to \frac {\cosh (k x+c_1)}{k}+c_2 \\ \end{align*}

✗ Sympy

from sympy import * 
x = symbols("x") 
k = symbols("k") 
y = Function("y") 
ode = Eq(-k**2*(Derivative(y(x), x)**2 + 1) + Derivative(y(x), (x, 2))**2,0) 
ics = {} 
dsolve(ode,func=y(x),ics=ics)
 

NotImplementedError : The given ODE Derivative(y(x), x) - sqrt((-k + Derivative(y(x), (x, 2)))*(k + Derivative(y(x), (x, 2))))/k cannot be solved by the factorable group method