  In this paper we are concerned with the existence of a weak solution to the
initial boundary value problem for the equation $\frac{\partial u}{\partial t}
= \Delta\left(\Delta u\right)^{-3}$. This problem arises in the mathematical
modeling of the evolution of a crystal surface. Existence of a weak solution
$u$ with $\Delta u\geq 0$ is obtained via a suitable substitution. Our
investigations reveal the close connection between this problem and the
equation $\partial_t\rho+\rho^2\Delta^2\rho^3=0$, another crystal surface model
first proposed by H. Al Hajj Shehadeh, R. V. Kohn and J. Weare in Physica D:
Nonlinear Phenomena, bf 240 (2011), no. 21, 1771-1784.
