ソリトン
∂ u ∂ t + 6 u ∂ u ∂ x + ∂ 3 u ∂ x 3 = 0 {\displaystyle {\frac {\partial u}{\partial t}}+6u{\frac {\partial u}{\partial x}}+{\frac {\partial ^{3}u}{\partial x^{3}}}=0}