ソリトン
d 2 u n d t 2 = e − ( u n − u n − 1 ) − e − ( u n + 1 − u n ) {\displaystyle {\frac {d^{2}u_{n}}{dt^{2}}}=e^{-(u_{n}-u_{n-1})}-e^{-(u_{n+1}-u_{n})}}