Abstract:
We present a new Bäcklund-type transformation for the nonlinear equation $\theta_{xy}=2\sqrt{\lambda(x,y)\theta_x\theta_y}$ studied by É. Goursat. Goursat found a linearization transformation and some properties of this equation, which make it similar to the Moutard equation $u_{xy}=M(x,y)u$. However, this Goursat transformation does not provide proper superposition formulas. We give the necessary extended superposition formulas.