# Particular solution of third order PDE

$$\partial_t u + \partial^{3}_x u = 0$$ show that this PDE has a particular solution in the form of a plane wave, $$u_k(x,t) = \exp(-i\,w\,t - i\,k\,x)$$ if the frequency $w$ obeys the appropriate dispersion relation, $w = w(k)$.

In the past I have solved up to 2nd order PDE's using separation of variables, Sturm-Liouville method. But since the all coefficients in the PDE has different powers I am confused how to solve this problem. Seems like I can not solve this problem using the methods I used to solve 2nd order PDE's. Any help?

• What happens when you plug this into the PDE. Can you show that it works, provided there is a certain relation (which you must find) between $w$ and $k$? – GEdgar Dec 12 '16 at 18:09

Since you have a linear PDE you can use the principle of superposition to form solutions with $$u \sim \exp(−iwt−ikx)$$ then inserting the plane wave solution you obtain $$-i\omega u + (-ik)^3u = \left(-i\omega-i^3k^3\right)u = -i(\omega -k^3)u = 0$$ or the dispersion relation is $$w(k) = k^3$$ if this relation is true then you can use the plane wave above.
• I get $w=k^3$. Here $i=\sqrt{-1}$ – Aleksas Domarkas Jun 4 '18 at 6:39