ソリトン
∂ 2 u ∂ t 2 − ∂ 2 u ∂ x 2 − 3 ∂ 2 u 2 ∂ x 2 − ∂ 4 u ∂ x 4 = 0 {\displaystyle {\frac {\partial ^{2}u}{\partial t^{2}}}-{\frac {\partial ^{2}u}{\partial x^{2}}}-3{\frac {\partial ^{2}u^{2}}{\partial x^{2}}}-{\frac {\partial ^{4}u}{\partial x^{4}}}=0}