Mathematics of paper folding

The art of origami or paper folding has received a considerable amount of mathematical study. Fields of interest include a given paper model's flat-foldability (whether the model can be flattened without damaging it) and the use of paper folds to solve mathematical equations.

Pure origami

Flat folding

Two-colorability.

Mountain-valley counting.

Angles around a vertex.

The construction of origami models is sometimes shown as crease patterns. The major question about such crease patterns is whether a given crease pattern can be folded to a flat model, and if so, how to fold them; this is an NP-complete problem.^[1] Related problems when the creases are orthogonal are called map folding problems. There are four mathematical rules for producing flat-foldable origami crease patterns:^[2]

1. crease patterns are two colorable
2. Maekawa's theorem: at any vertex the number of valley and mountain folds always differ by two in either direction
3. Kawasaki's theorem: at any vertex, the sum of all the odd angles adds up to 180 degrees, as do the even.
4. a sheet can never penetrate a fold.

Paper exhibits zero Gaussian curvature at all points on its surface, and only folds naturally along lines of zero curvature. Curved surfaces which can't be flattened can be produced using a non-folded crease in the paper, as is easily done with wet paper or a fingernail.

Assigning a crease pattern mountain and valley folds in order to produce a flat model has been proven by Marshall Bern and Barry Hayes to be NP complete.^[3] Further references and technical results are discussed in Part II of Geometric Folding Algorithms.^[4]

Axioms

Some classical construction problems of geometry — namely trisecting an arbitrary angle, or doubling the cube — are proven to be unsolvable using compass and straightedge, but can be solved using only a few paper folds.^[5] Paper fold strips can be constructed to solve equations up to degree 4. (The Huzita–Hatori axioms are one important contribution to this field of study.) Complete methods for solving all equations up to degree 4 by applying such methods are discussed in detail in Geometric Origami.^[6]

As a result of origami study through the application of geometric principles, methods such as Haga's theorem have allowed paperfolders to accurately fold the side of a square into thirds, fifths, sevenths, and ninths. Other theorems and methods have allowed paperfolders to get other shapes from a square, such as equilateral triangles, pentagons, hexagons, and special rectangles such as the golden rectangle and the silver rectangle. Methods for folding most regular polygons up to and including the regular 19-gon have been developed.^[6]

Constructions

Haga's theorems

BQ is always a rational if AP is.

The side of a square can be divided at an arbitrary rational fraction in a variety of ways. Haga's theorems say that a particular set of constructions can be used for such divisions.^[7] Surprisingly few folds are necessary to generate large odd fractions. For instance ¹⁄₅ can be generated with three folds; first halve a side, then use Haga's theorem twice to produce first ²⁄₃ and then ¹⁄₅.

The accompanying diagram shows Haga's first theorem:

$BQ = \frac{2 AP}{1 + AP}$

Interestingly the function changing the length AP to QC is self inverse. Let x be AP then a number of other lengths are also rational functions of x. For example:

Haga's first theorem

AP	BQ	QC	AR	PQ
x	$\frac{2 x}{1+x}$	$\frac{1-x}{1+x}$	$\frac{1-x^2}{2}$	$\frac{1+x^2}{1+x}$
1⁄2	2⁄3	1⁄3	3⁄8	5⁄6
1⁄3	1⁄2	1⁄2	4⁄9	5⁄6
2⁄3	4⁄5	1⁄5	5⁄18	13⁄15
1⁄5	1⁄3	2⁄3	12⁄25	13⁄15

Doubling the cube

Doubling the cube: PB/PA = cube root of 2

The classical problem of doubling the cube can be solved by first creasing a square of paper into three equal strips as shown in the diagram. Then the bottom edge is positioned so the corner point P is on the top edge and the crease mark on the edge meets the other crease mark Q. The length PB will then be the cube root of 2 times the length of AP.^[8]

The edge with the crease mark is considered a marked straightedge, something which is not allowed in compass and straightedge constructions. Using a marked straightedge in this way is called a neusis construction in geometry.

Related problems

The problem of rigid origami, treating the folds as hinges joining two flat, rigid surfaces, such as sheet metal, has great practical importance. For example, the Miura map fold is a rigid fold that has been used to deploy large solar panel arrays for space satellites.

The napkin folding problem is the problem of whether a square or rectangle of paper can be folded so the perimeter of the flat figure is greater than that of the original square.

Curved origami also poses a (very different) set of mathematical challenges.^[9] Curved origami allows the paper to form developable surfaces that are not flat.

Wet-folding origami allows an even greater range of shapes.

The maximum number of times an incompressible material can be folded has been derived. With each fold a certain amount of paper is lost to potential folding. The loss function for folding paper in half in a single direction was given to be $L=\tfrac{\pi t}{6} (2^n + 4)(2^n - 1)$, where L is the minimum length of the paper (or other material), t is the material's thickness, and n is the number of folds possible. The distances L and t must be expressed in the same units, such as inches. This function was derived by Britney Gallivan in 2001 (then only a high school student) who then folded a sheet of paper in half 12 times, contrary to the popular belief that paper of any size could be folded at most eight times. She also derived the equation for folding in alternate directions.^[10]

The fold-and-cut problem asks what shapes can be obtained by folding a piece of paper flat, and making a single straight complete cut. The solution, known as the Fold and Cut Theorem, states that any shape with straight sides can be obtained.

See also

Origami portal

• Napkin folding problem
• Map folding
• Regular paperfolding sequence (for example, the Dragon curve)

Notes

1. ^ Thomas C. Hull (2002). "The Combinatorics of Flat Folds: a Survey". The Proceedings of the Third International Meeting of Origami Science, Mathematics, and Education. AK Peters. ISBN 9781568811819. http://kahuna.merrimack.edu/~thull/papers/flatsurvey.pdf. 
2. ^ "Robert Lang folds way-new origami". http://www.ted.com/index.php/talks/robert_lang_folds_way_new_origami.html. 
3. ^ The Complexity of Flat Origami
4. ^ Demaine, Erik; O'Rourke, Joseph (July 2007). Geometric Folding Algorithms: Linkages, Origami, Polyhedra. Cambridge University Press. ISBN 978-0-521-85757-4. http://www.gfalop.org 
5. ^ Origami Geometric Constructions
6. ^ ^{a b} Geretschläger, Robert (2008). Geometric Origami. UK: Arbelos. ISBN 978-0-9555477-1-3. http://www.arbelos.co.uk/GeometricOrigami.html. 
7. ^ Koshiro. "How to Divide the Side of Square Paper". Japan Origami Academic Society. http://origami.gr.jp/Archives/People/CAGE_/divide/02-e.html. 
8. ^ Peter Messer (1986). "Problem 1054". Crux Mathematicorum 12 (10): pp. 284–285.. 
9. ^ Siggraph: "Curved Origami"
10. ^ Weisstein, Eric W., "Folding" from MathWorld.

Further reading

• Robert J. Lang (2003). Origami Design Secrets: Mathematical Methods for an Ancient Art. A K Peters. ISBN 1568811942. 
• Haga, Kazuo (2008). Fonacier, Josefina C; Isoda, Masami. eds. Origamics: Mathematical Explorations Through Paper Folding. University of Tsukuba, Japan: World Scientific Publishing. ISBN 978-9812834904 

External links

• Dr. Tom Hull. "Origami Mathematics Page". http://mars.wnec.edu/~th297133/origamimath.html 
• Paper Folding Geometry at cut-the-knot
• Dividing a Segment into Equal Parts by Paper Folding at cut-the-knot
• Britney Gallivan has solved the Paper Folding Problem