1. Darboux, G., Le-cons sur la the-orie ge-ne-rale des surfaces et les applications ge-ome-triques du calcul infinite-simal, Paris: Gauthier-Villars, 1889, vol. 2.
2. Zhiber, A.V. and Startsev, S.Ya., Integrals, Solutions, and Existence Problems for Laplace Transformations of Linear Hyperbolic Systems, Math. Notes, 2003, vol. 74, nos. 5–6, pp. 803–811.
3. Athorne, C.A., A Z 2 × R 3 Toda System, Phys. Lett. A, 1995, vol. 206, pp. 162–166.
4. Petrén, L., Extension de la méthode de Laplace aux équations $$\sum\nolimits_{i = 0}^{n - 1} {A_{1i} \frac{{\partial ^{i + 1} z}} {{\partial x\partial y^i }}} + \sum\nolimits_{i = 0}^n {A_{0i} \frac{{\partial ^i z}} {{\partial y^i }}}$$ , Lund Univ. Arsskrift, 1911, vol. 7, no. 3, pp. 1–166.
5. Pisati, L., Sulla estensione del metodo di Laplace alle equazioni con due variabili, Rend. Circ. Mat. Palermo, vol. 20, pp. 344–374.