hi there, i have the following pde: $$(\partial_x x^4 \partial_x  \partial_t^2)y(x,t)=0$$ and found the solution $$y=a+t^21/x^2$$, with a a constant. Is this solution unique? Does anyone know of any other solution, tricks to generate other solutions? Thankful for help. Regina

Since your equation is linear and homogeneous, linear combinations of solutions are solutions. Basic solutions include $1$ and $t^2  1/x^2$ from your solution, as well as $1/x^3$, $t$, and $(1a/x) \exp(a/x) \exp(a t)$ and $(1b/x) \exp(b/x) \exp(b t)$ for any constants a and b. 


PDEs look like ODEs, but only look like. The solution set of an ODE of order $n$ is usually parametrized by $n$ scalar (integration constant). On the contrary, the solution set of a PDE of order $n$ in $d$ independent variables ($d=2$ in your case) is usually parametrized by $n$ functions of $d1$ variables. This is clear in the hyperbolic case because you just solve a Cauchy problem with initial data on a noncharacteristic hypersurface. More generally, if the equation has analytic coefficients, you can apply the CauchyKowalevskaia Theorem. In conclusion, your explicit solutions are far from unique. 


More generally, $F(t + 1/x)  x F'(t + 1/x)$ and $F(t  1/x) + x F'(t  1/x)$ for any differentiable function F. 

