I was writing some exercises about the AM-GM inequality and I got carried away by the following (pretty nontrivial, I believe) question:

Q:By properly folding a common 210mm×297mm sheet of paper, what

is the maximum amount of water such a sheet is able to contain?The volume of the optimal

box(on the right) is about 1.128l. But the volume of thebutterfly(in my left hand) seems to be much bigger and I am not sure at all about the shape of the optimal folded sheet. Is is something boat-like?Clarifications: we may assume to have a magical glue to prevent water from leaking through the cracks, or for glueing together points of the surface. Solutions where parts of the sheet are cut out,

thenglued back together deserve to be considered as separate cases. On the other hand these cases are trivial, as pointed by joriki in the comments below. The isoperimetric inequality gives that the maximum volume is <2.072l.As pointed out by Rahul, here it is a way for realizing the optimal configuration: the maximum capacity of the following A4+A4 bag exceeds 2.8l.

**Answer**

This problem reminds me of tension field theory and related problems in studying the shape of inflated inextensible membranes (like helium balloons). What follows is far from a solution, but some initial thoughts about the problem.

First, since you're allowing creasing and folding, by Nash-Kuiper it's enough to consider short immersions

ϕ:P⊂R2→R3,‖dϕTdϕ‖2≤1

of the piece of paper P into R3, the intuition being that you can always "hide" area by adding wrinkling/corrugation, but cannot "create" area. It follows that we can assume, without loss of generality, that ϕ sends the paper boundary ∂P to a curve γ in the plane.

We can thus partition your problem into two pieces: (I) given a fixed curve γ, what is the volume of the volume-maximizing surface Mγ with ϕ(∂P)=γ? (II) Can we characterize γ for which Mγ has maximum volume?

Let's consider the case where γ is given. We can partition Mγ into

1) regions of pure tension, where dϕTdϕ=I; in these regions Mγ is, by definition, developable;

2) regions where one direction is in tension and one in compression, ‖dϕTdϕ‖2=1 but det.

We need not consider \|d\phi^Td\phi\|_2 < 1 as in such regions of pure compression, one could increase the volume while keeping \phi a short map.

Let us look at the regions of type (2). We can trace on these regions a family of curves \tau along which \phi is an isometry. Since M_{\gamma} maximizes volume, we can imagine the situation physically as follows: pressure inside M_{\gamma} pushes against the surface, and is exactly balanced by stress along inextensible fibers \tau. In other words, for some stress \sigma constant along each \tau, at all points \tau(s) along \tau we have

\hat{n} = \sigma \tau''(s)

where \hat{n} the surface normal; it follows that (1) the \tau follow geodesics on M_{\gamma}, (2) each \tau has constant curvature.

The only thing I can say about problem (II) is that for the optimal \gamma, the surface M_\gamma must meet the plane at a right angle. But there are many locally-optimal solutions that are not globally optimal (for example, consider a half-cylinder (type 1 region) with two quarter-spherical caps (type 2 region); it has volume \approx 1.236 liters, less than Joriki's solution).

I got curious so I implemented a quick-and-dirty tension field simulation that optimizes for \gamma and M_{\gamma}. Source code is here (needs the header-only Eigen and Libigl libraries): https://github.com/evouga/DaurizioPaper

Here is a rendering of the numerical solution, from above and below (the volume is roughly 1.56 liters).

EDIT 2: A sketch of the orientation of \tau on the surface:

**Attribution***Source : Link , Question Author : Jack D'Aurizio , Answer Author : user7530*