I wonder if the ODE


$y''+e^{y}=a$ 


can be solved explicitly. For $a=0$, it is well-known that there is a two-parameter family of explicit solutions 


$y=\ln(2)-2\ln(\cosh(cx+d))+2\ln(c)$, $c,d \in R$. Are there explicit solutions for $a>0$? 



Explicit solution for one-dimensional Gelfand problem
Multiply by $y'$, we get $((y')^2/2+e^y-ay)'=0$, so $(y')^2/2+e^y-ay=c$, $y'=f(y)$, where $f(y)=\pm \sqrt{2c+2ay-2e^y}$, so $dx/dy=1/f(y)$, $x$ is antiderivative of $1/f(y)$. I doubt that this antiderivative is expressed in elementary functions for general $a,c$.


