Lagrange,s Equation
The partial differential equation
(Gray 1997, p. 399), whose solutions are called minimal surfaces. This corresponds to the mean curvature 
equalling 0 over the surface.
d'Alembert's equation
is sometimes also known as Lagrange's equation (Zwillinger 1997, pp. 120 and 265-268).
REFERENCES:
do Carmo, M. P. "Minimal Surfaces." §3.5 in Mathematical Models from the Collections of Universities and Museums (Ed. G. Fischer). Braunschweig, Germany: Vieweg, pp. 41-43, 1986.
Gray, A. Modern Differential Geometry of Curves and Surfaces with Mathematica, 2nd ed. Boca Raton, FL: CRC Press, 1997.
Zwillinger, D. "Lagrange's Equation." §II.A.69 in Handbook of Differential Equations, 3rd ed. Boston, MA: Academic Press, pp. 120 and 265-268, 1997.
