ソリトン
i ∂ u ∂ t + 1 2 ∂ 2 u ∂ x 2 + κ | u | 2 u = 0 {\displaystyle i{\frac {\partial u}{\partial t}}+{\frac {1}{2}}{\frac {\partial ^{2}u}{\partial x^{2}}}+\kappa |u|^{2}u=0}