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A pair of linear Operators 
 and 
 associated with a given Partial Differential Equation
which can be used to solve the equation.  However, it turns out to be very difficult to find the 
 and 
 corresponding
to a given equation, so it is actually simpler to postulate a given 
 and 
 and determine to which Partial
Differential Equation they correspond (Infeld and Rowlands 1990).
See also Partial Differential Equation
References
Infeld, E. and Rowlands, G.  ``Integrable Equations in Two Space Dimensions as Treated by the Zakharov Shabat Methods.''
  §7.10 in Nonlinear Waves, Solitons, and Chaos.  Cambridge, England: Cambridge University Press, pp. 216-223, 1990.