文摘
It is demonstrated that the fourth-order PDE<math>\left| \matrix{f_{xxxx} & f_{xxxt} & f_{xxtt} \cr f_{xxxt}& f_{xxtt}& f_{xttt} \cr f_{xxtt}& f_{xttt}& f_{tttt}}\right| = 0math>decouples in a pair of identical second-order Monge–Ampère equations,<math>u_{xx}u_{tt} - u_{xt}^2 = 0 \quad {\rm and} \quad {v}_{xx}{v}_{tt} - {v}_{xt}^2 = 0,math>by virtue of the Bäcklund-type relation<math>{\rm d}^4f={({\rm d}^2u)^2 \over u_{xx}}+ {({\rm d}^2{v})^2\over{v}_{xx}}.math>A higher order generalization of this decomposition ia also proposed.