Bäcklund transformations for the equation ∂2u/∂x1∂x1+∂2u/∂x2∂x2=f(u) is an arbitrary function) is studied in this paper, using the procedure of Wahlquist and Estabrook (WEP). We conclude that the condition d2f/du2=λf is sufficient for the existence of Bäcklund transformations for the equation of our interest. A special case of our results leads to the conclusion of Leibbrandt[1,2].
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Rogers,C.and W.F.Shadwick,Backlund Transformations and Their Applications,Academic Press(1982).
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Segur,H.,Some open problems,Physica,18D(1986),1-12.