  A preliminary group classification of the class 2D nonlinear heat equations
$u_t=f(x,y,u,u_x,u_y)(u_{xx}+u_{yy})$, where $f$ is arbitrary smooth function
of the variables $x,y,u,u_x$ and $u_y$ using Lie method, is given. The paper is
one of the few applications of an algebraic approach to the problem of group
classification: the method of preliminary group classification.
