Surface equivalent of catenary curve A catenary curve
is the shape taken by an idealized hanging chain or rope under the influence
of gravity.  It has the equation $y= a \cosh (x/a)$.
My question is:

What is the shape taken by an idealized, thin two-dimensional sheet,
  pinned on a plane parallel to the ground, under the influence of gravity?

The answer surely depends on how it is pinned to the plane, the boundary
conditions.
Natural options are:


*

*A disk sheet fixed to a circle.

*A square sheet fixed to a square.

*A square sheet pinned at its four corners.


The middle option above would look something like this when inverted:

          


          

(Image by Tim Tyler at hexdome.com.)


I don't think any of these shapes is a
catenoid,
which is the surface of revolution formed by a catenary curve.
Is there a simple analytic description of any of these surfaces,
analogous to the $\cosh$ equation for the catenary curve?
I have been unsuccessful in finding anything but simulations of solutions
of the differential equations.
This question arose in imagining a higher-dimensional version
of the property that an inverted catenary supports smooth rides of a square-wheeled
bicycle
(explored in this MO question).
Thanks for pointers!
 A: The thing that comes to mind is the capillary surface including gravity. See the note by Finn, available free as a pdf, as a reference at the end of:
http://en.wikipedia.org/wiki/Capillary_surface 
Hmmm, maybe not. Your surface would not have a large flat region in the middle...
A rotated catenary surface is quite simply not isometric to a flat disc. So we might, for instance, be asking about a rubber sheet, glued down along a  boundary, and allowed to sag in the middle under gravity. The elastic energy less resembles the mean curvature operator in favor of the ordinary Laplacian 
http://en.wikipedia.org/wiki/Elastic_energy 
It appears you are looking for the biharmonic equation, as the force of gravity vector field will be considered constant and divergence free, so the displacement $u$ satisfies
$\Delta^2 u = 0.$ See 
http://en.wikipedia.org/wiki/Linear_elasticity#Elastostatics 
A: A model equation for an inextensible, ﬂexible, heavy surface  in a gravitational ﬁeld was deduced by Poisson Lagrange and later the problem was  also studied by Poisson (see the references in the linked papers below). The equilibrium condition for a hanging heavy surface of constant mass density reads 
$$\sqrt{1+|\nabla u|^2}\ \nabla\cdot{}\frac{\nabla u}{\sqrt{1+|\nabla u|^2}}=\frac{1}{u+\lambda},\qquad x\in\Omega\subset\mathbb R^2,\qquad\qquad(1)$$
where $u=u(x)$ is the vertical displacement and $\lambda\in\mathbb R$ is an arbitrary constant (a Lagrange multiplier). (1) is the Euler equation of the variational integral
$$I(u)=\int_{\Omega}u\sqrt{1+|\nabla u|^2}dx,$$
which can be interpreted as the vertical coordinate of the center of gravity of the surface
$$\mbox{graph}(u)=\{(x,u(x)):\ x\in\Omega\}\subset\mathbb R^2\times\mathbb R.$$ 
Equation (1) is to be supplemented with the requirement that the surface has a prescribed area $A$
$$\qquad\qquad\qquad\qquad\qquad\int_{\Omega}\sqrt{1+|\nabla u|^2}dx=A,\qquad\qquad\qquad\qquad\qquad\qquad\quad\quad(2) $$
and the Dirichlet boundary condition describing the curve from which the surface is being suspended
$$\qquad\qquad\qquad\qquad\qquad\qquad\qquad\quad\left.u\right|_{\partial \Omega}=g.\qquad\qquad\qquad\qquad\qquad\qquad\qquad\qquad\quad(3) $$
One can check formally that a solution to (1)-(3) provides a graph of a heavy surface of prescribed area and boundary with the lowest center of gravity, so this is a precise 2D analogue of the classical catenary problem. 
It is known that problem (1)-(3) has no classical solutions for the values of area $A$ outside of some bounded interval $[A_{\min},A_{\max}]$. Moreover, the corresponding variational problem has no global solutions for all $A\in\mathbb R$. A short survey of some old and relatively new results concerning well-posedness of (1)-(3) and its multidimensional analogues can be found in the paper by Dierkes and Huisken, "The N-dimensional analogue of the catenary: Prescribed area", in J. Jost (ed) Calculus of Variations and Geometric Analysis, Int. Press (1996), pp. 1-13.
Addendum. Here is a more recent survey by Dierkes: "Singular Minimal Surfaces" (in Geometric Analysis and Nonlinear Partial Differential Equations, Springer (2003),  pp. 177-194). 
