“…Furthermore, we supplement p = u x and q = u y to obtain a first order strip C 1 . If by using the PDE (3) and the strip conditions (9) we are able to determine r = u x x , s = u xy and t = u yy uniquely, then the strip C 2 = {(x(λ), y(λ), u(λ), p(λ), q(λ), r (λ), s(λ), t(λ)) | λ ∈ I } is called an integral strip, and C 1 is a free strip, otherwise C 1 is called a characteristic strip. Note that along a free strip, but not along a characteristic strip, the derivatives u x x , u xy and u yy can all be determined along the strip, either by being interior derivatives with respect to C 1 , or by combining the PDE (3) with the remaining interior derivatives.…”