Square

This article is about the shape. For other uses, see Square (disambiguation).

Square
Type	quadrilateral regular polygon
Edges and vertices	4
Symmetry group	order-8 dihedral
Area	side2
Internal angle (degrees)	π/2 (90°)
Perimeter	4 · side

In geometry, a square is a regular quadrilateral, which means that it has four straight sides of equal length and four equal angles. Squares are special cases of rectangles, which have four equal angles, and of rhombuses, which have four equal sides. As with all rectangles, a square's angles are right angles (90 degrees, or π/2 radians), making adjacent sides perpendicular. The area of a square is the side length multiplied by itself, and so in algebra, multiplying a number by itself is called squaring.

Equal squares can tile the plane edge-to-edge in the square tiling. Square tilings are ubiquitous in tiled floors and walls, graph paper, image pixels, and game boards. Square shapes are also often seen in building floor plans, origami paper, food servings, in graphic design and heraldry, and in instant photos and fine art.

The formula for the area of a square forms the basis of the calculation of area and motivates the search for methods for squaring the circle by compass and straightedge, now known to be impossible. Squares can be inscribed in any smooth or convex curve such as a circle or triangle, but it remains unsolved whether a square can be inscribed in every simple closed curve. Several problems of squaring the square involve subdividing squares into unequal squares. Mathematicians have also studied packing squares as tightly as possible into other shapes.

Squares form the metric balls for taxicab geometry and Chebyshev distance, two forms of non-Euclidean geometry. Although spherical geometry and hyperbolic geometry both lack polygons with four equal sides and right angles, they have square-like regular polygons with four sides and other angles, or with right angles and different numbers of sides.

Definitions

Among rectangles (top row), the square is the shape with equal sides (blue, middle). Among rhombuses (bottom row), the square is the shape with right angles (blue, middle).

Squares can be defined in many equivalent ways. If a quadrilateral (a four-sided polygon in the Euclidean plane) satisfies any one of the following definitions, it satisfies all of them:

• A square is a rectangle with four equal sides. ^[1]
• A square is a rhombus with a right angle between a pair of adjacent sides. ^[1]
• A square is a rhombus with all angles equal. ^[1]
• A square is a parallelogram with one right angle and two adjacent equal sides. ^[1]
• A square is a quadrilateral with four equal sides and four right angles; that is, it is a quadrilateral that is both a rhombus and a rectangle ^[1]
• A square is a quadrilateral where the diagonals are equal, and are the perpendicular bisectors of each other. That is, it is a rhombus with equal diagonals. ^[2]
• A square is a quadrilateral with successive sides ${\displaystyle a}$, ${\displaystyle b}$, ${\displaystyle c}$, ${\displaystyle d}$ whose area is ^[3] $${\displaystyle A={\frac {1}{4}}(a^{2}+b^{2}+c^{2}+d^{2}).}$$

Squares are the only regular polygons whose internal angle, central angle, and external angle are all equal (they are all right angles). ^[4]

Properties

A square is a special case of a rhombus (equal sides, opposite equal angles), a kite (two pairs of adjacent equal sides), a trapezoid (one pair of opposite sides parallel), a parallelogram (all opposite sides parallel), a quadrilateral or tetragon (four-sided polygon), and a rectangle (opposite sides equal, right-angles), ^[1] and therefore has all the properties of all these shapes, namely:

• All four internal angles of a square are equal (each being 90°, a right angle). ^{[4] [5]}
• The central angle of a square is equal to 90°. ^[4]
• The external angle of a square is equal to 90°. ^[4]
• The diagonals of a square are equal and bisect each other, meeting at 90°. ^[5]
• The diagonals of a square bisect its internal angles, forming adjacent angles of 45°. ^[6]
• All four sides of a square are equal. ^[7]
• Opposite sides of a square are parallel. ^[8]

All squares are similar to each-other, meaning they have the same shape, ^[9] and one parameter (typically the length of a side or diagonal) ^[10] suffices to specify a square's size; squares of the same size are congruent. ^[11]

Measurement

The area of a square is the product of the lengths of its sides.

A square whose four sides have length ${\displaystyle \ell }$ has perimeter ^[12] ${\displaystyle P=4\ell }$ and diagonal length ${\displaystyle d={\sqrt {2}}\ell }$. ^[13] (The square root of 2, appearing in this formula, is irrational, meaning that it is not the ratio of any two integers. It is approximately equal to 1.414. ^[14] ) A square's area is ^[13] $${\displaystyle A=\ell ^{2}={\tfrac {1}{2}}d^{2}.}$$ This formula for the area of a square as the second power of its side length led to the use of the term squaring to mean raising any number to the second power. ^[15] Reversing this relation, the side length of a square of a given area is the square root of the area. Squaring an integer, or taking the area of a square with integer sides, results in a square number; these are figurate numbers representing the numbers of points that can be arranged into a square grid. ^[16]

Since four squared equals sixteen, a four by four square has an area equal to its perimeter. That is, it is an equable shape. The only other integer rectangle with such a property is a three by six rectangle. ^[17]

Because it is a regular polygon, a square is the quadrilateral of least perimeter enclosing a given area. Dually, a square is the quadrilateral containing the largest area within a given perimeter. ^[18] Indeed, if A and P are the area and perimeter enclosed by a quadrilateral, then the following isoperimetric inequality holds: $${\displaystyle 16A\leq P^{2}}$$ with equality if and only if the quadrilateral is a square. ^{[19] [20]}

Symmetry

Classification of quadrilaterals by their symmetry subgroups. The 8-fold symmetry of the square is labeled as r8, at the top of the image. The "gyrational square" below it corresponds to the subgroup of four orientation-preserving symmetries of a square, using rotations but not reflections.

The square is the most symmetrical of the quadrilaterals. Like all regular polygons, it is an isogonal figure, meaning that it has symmetries taking every vertex to every other vertex, and an isotoxal figure, meaning that it has symmetries taking every edge to every other edge. More strongly, the symmetries of the square and of any other regular polygon act transitively on the flags of the polygon, pairs of a vertex and edge that touch each other. This means that there is a symmetry taking each of the eight flags of the square to each other flag. ^[22]

There are eight congruence transformations of the plane that take the square to itself: ^[23]

• Leaving the square unchanged (the identity transformation)
• Rotation around the center of the square by 90°, 180°, or 270°
• Reflection across a diagonal, or across a centerline of the square, parallel to one of its sides.

Combining any two of these transformations by performing one after the other produces another symmetry. This operation on pairs of symmetries gives the eight symmetries of a square the mathematical structure of a point group, the dihedral group of order eight. ^[23]

p4, Egyptian tomb ceiling

p4m, Nineveh & Persia

p4g, China

Wallpaper groups of tilings from The Grammar of Ornament

The wallpaper groups are symmetry groups of two-dimensional repeating patterns. For many of these groups the basic unit of repetition (the unit cell of its period lattice) can be a square, and for three of these groups, p4, p4m, and p4g, it must be a square. ^[24]

Inscribed and circumscribed circles

The inscribed circle (purple) and circumscribed circle (red) of a square (black)

The inscribed circle of a square is the largest circle that can fit inside that square. Its center is at the center point of the square, and its radius (the inradius of the square) has length ${\displaystyle r=\ell /2}$. The inscribed circle touches the sides of the square at their midpoints; because it touches all four sides, the square is a tangential quadrilateral. The circumscribed circle of a square is the circle passing through the four vertices, making the square a cyclic quadrilateral. Its radius, known as the circumradius, has length ${\displaystyle R=\ell /{\sqrt {2}}}$. ^[25] If the inscribed circle of a square ${\displaystyle ABCD}$ has tangency points ${\displaystyle E}$ on ${\displaystyle AB}$, ${\displaystyle F}$ on ${\displaystyle BC}$, ${\displaystyle G}$ on ${\displaystyle CD}$, and ${\displaystyle H}$ on ${\displaystyle DA}$, then for any point ${\displaystyle P}$ on the inscribed circle, ^[26] $${\displaystyle 2(PH^{2}-PE^{2})=PD^{2}-PB^{2}.}$$ If ${\displaystyle d_{i}}$ is the distance from an arbitrary point in the plane to the ${\displaystyle i}$th vertex of a square and ${\displaystyle R}$ is the circumradius of the square, then ^[27] $${\displaystyle {\frac {d_{1}^{4}+d_{2}^{4}+d_{3}^{4}+d_{4}^{4}}{4}}+3R^{4}=\left({\frac {d_{1}^{2}+d_{2}^{2}+d_{3}^{2}+d_{4}^{2}}{4}}+R^{2}\right)^{2}.}$$ If ${\displaystyle L}$ and ${\displaystyle d_{i}}$ are the distances from an arbitrary point in the plane to the centroid of the square and its four vertices respectively, then $${\displaystyle d_{1}^{2}+d_{3}^{2}=d_{2}^{2}+d_{4}^{2}=2(R^{2}+L^{2})}$$ and $${\displaystyle d_{1}^{2}d_{3}^{2}+d_{2}^{2}d_{4}^{2}=2(R^{4}+L^{4}),}$$ where ${\displaystyle R}$ is the circumradius of the square. ^[28]

Applications

Square tiles

Pixelated Girl with a Pearl Earring

Squares are so well-established as the shape of tiles that the Latin word tessera, for a small tile as used in mosaics, takes its derivation from a variant of the ancient Greek word for the number four, referring to the four corners of a square tile. ^[29] Graph paper, preprinted with a square tiling, has been widely used for data visualization using Cartesian coordinates ^[30] since its invention in 1794. ^[31] The pixels of bitmap images, as recorded by image scanners and digital cameras or displayed on electronic visual displays, conventionally lie at the intersections of a square grid, and are often considered as small squares, arranged in a square tiling. ^{[32] [33]} Many standard techniques for image compression and video compression, including the JPEG format, are based on the subdivision of images into larger square blocks of pixels. ^[34] The quadtree data structure used in data compression and computational geometry is based on the recursive subdivision of squares into smaller squares. ^[35]

Site of the Yongning Pagoda

Many architectural structures from both ancient and modern cultures have featured a square floor plan, base, or footprint. Ancient examples include the Egyptian pyramids, ^[36] Mesoamerican pyramids such as those at Teotihuacan, ^[37] the Chogha Zanbil ziggurat in what is now Iran, ^[38] the four-fold design of Persian walled gardens, said to model the four rivers of Paradise, and later structures inspired by their design such as the Taj Mahal in India, ^[39] the square bases of many Buddhist stupas, ^[40] and East Asian pagodas, buildings that symbolically face to the four points of the compass and reach to the heavens. ^[41] Norman keeps such as the Tower of London often take the form of a low square tower. ^[42] In modern architecture, a majority of skyscrapers feature a square plan for pragmatic rather than aesthetic or symbolic reasons. ^[43]

A Tibetan mandala

Broadway Boogie Woogie , Piet Mondrian

On a smaller scale, the stylized nested squares of a Tibetan mandala, like the design of a stupa, function as a miniature model of the cosmos. ^[44] Some formats for film photography use a square aspect ratio, notably many Polaroid cameras, medium format cameras, and Kodak Instamatic cameras. ^{[45] [46]} Artists whose works have used square frames and forms include Josef Albers, ^[47] Kazimir Malevich ^[48] and Piet Mondrian. ^[49]

16th-century Indian chessboard

Ostomachion

1615 horoscope

Baseball diamonds ^[50] and modern boxing rings are square despite being named for other shapes. ^[51] The square go board is said to represent the earth, with the 361 crossings of its lines representing days of the year. ^[52] The chessboard inherited its square shape from a pachisi-like Indian race game and in turn passed it on to checkers. ^[53] In two ancient games from Mesopotamia and Ancient Egypt, the Royal Game of Ur and Senet, the game board itself is not square, but rectangular, subdivided into a grid of squares. ^[54] The ancient Greek Ostomachion puzzle (according to some interpretations) involves rearranging the pieces of a square cut into smaller polygons, as does the Chinese tangram. ^[55] Another set of puzzle pieces, the polyominos, are formed from squares glued edge-to-edge. ^[56] Medieval and Renaissance horoscopes were often arranged in a square format, across Europe, the Middle East, and China. ^[57] Other recreational uses of squares include the shape of origami paper, ^[58] and a common style of quilting involving the use of square quilt blocks. ^[59]

Square flag of the municipality of Vuadens, based on the Swiss flag

QR code for the mobile English Wikipedia

Square waffles

Squares are a common element of graphic design, often used to give a sense of stability, symmetry, and order. ^[60] In heraldry, a canton (a design element in the top left of a shield) is normally square, and a square flag is called a banner. ^[61] The flag of Switzerland is square, as are the flags of the Swiss cantons. ^[62] QR codes are square and feature prominent nested square alignment marks in three corners. ^[63] Robertson screws have a square drive socket. ^[64] Crackers and sliced cheese are often square, ^[65] as are waffles. ^{[66] [67]} Square foods named for their square shapes include caramel squares, date squares, lemon squares, ^[68] square sausage, ^[69] and Carré de l'Est cheese. ^[70]

Constructions

Coordinates and equations

${\displaystyle |x|+|y|=2}$ plotted on Cartesian coordinates .

A unit square is a square of side length one. Often it is represented in Cartesian coordinates as the square enclosing the points ${\displaystyle (x,y)}$ that have ${\displaystyle 0\leq x\leq 1}$ and ${\displaystyle 0\leq y\leq 1}$. ^[71]

An axis-parallel square with its center at the point ${\displaystyle (x_{c},y_{c})}$ and sides of length ${\displaystyle 2r}$ (where ${\displaystyle r}$ is the inradius, half the side length) has vertices at the four points ${\displaystyle (x_{c}\pm r,y_{c}\pm r)}$. Its interior consists of the points ${\displaystyle (x,y)}$ with ${\displaystyle \max(|x-x_{c}|,|y-y_{c}|)<r}$, and its boundary consists of the points with ${\displaystyle \max(|x-x_{c}|,|y-y_{c}|)=r}$. ^[72]

A diagonal square with its center at the point ${\displaystyle (x_{c},y_{c})}$ and diagonal of length ${\displaystyle 2R}$ (where ${\displaystyle R}$ is the circumradius, half the diagonal) has vertices at the four points ${\displaystyle (x_{c}\pm R,y_{c})}$ and ${\displaystyle (x_{c},y_{c}\pm R)}$. Its interior consists of the points ${\displaystyle (x,y)}$ with ${\displaystyle |x-x_{c}|+|y-y_{c}|<R}$, and its boundary consists of the points with ${\displaystyle |x-x_{c}|+|y-y_{c}|=R}$. ^[72] For instance the illustration shows a diagonal square centered at the origin ${\displaystyle (0,0)}$ with circumradius 2, given by the equation ${\displaystyle |x|+|y|=2}$.

A square with Gaussian integer vertices can be characterized by its center c and half-diagonal p. Either c and p will both be Gaussian integers, or (as in this case) both be Gaussian integers offset by ⁠1/2⁠(1 + i).

In the plane of complex numbers, multiplication by the imaginary unit ${\displaystyle i}$ rotates the other term in the product by 90° around the origin (the number zero). Therefore, if any nonzero complex number ${\displaystyle p}$ is repeatedly multiplied by ${\displaystyle i}$, giving the four numbers ${\displaystyle p}$, ${\displaystyle ip}$, ${\displaystyle -p}$, and ${\displaystyle -ip}$, these numbers will form the vertices of a square centered at the origin. ^[73] If one interprets the real part and imaginary part of these four complex numbers as Cartesian coordinates, with ${\displaystyle p=x+iy}$, then these four numbers have the coordinates ${\displaystyle (x,y)}$, ${\displaystyle (-y,x)}$, ${\displaystyle (-x,-y)}$, and ${\displaystyle (-y,-x)}$. ^[74] Addition of complex numbers corresponds to translation of the complex plane, ^[75] so this origin-centered square can be translated to have any other center ${\displaystyle c}$ by adding the complex number ${\displaystyle c}$ to each of its four vertices. Alternately, taking the origin and the complex number ${\displaystyle s}$ as two of its vertices, a square can be formed of the vertices ${\displaystyle \{0,s,{}}$${\displaystyle s+is,is\}}$.^{[ original research? ][ citation needed ]}

The Gaussian integers, complex numbers with integer real and imaginary parts, form a square lattice in the complex plane. ^[75] If a square's vertices are all Gaussian integers, they can be written as ${\displaystyle \{c+p,c+ip,{}}$${\displaystyle c-p,c-ip\}}$, in terms of the center ${\displaystyle c}$ and half-diagonal ${\displaystyle p}$, which will either both be Gaussian integers or both be Gaussian integers offset by ${\displaystyle {\tfrac {1}{2}}(1+i)}$ (that is, complex numbers of the form ${\displaystyle a+{\tfrac {1}{2}}+(b+{\tfrac {1}{2}})i}$, for integers ${\displaystyle a}$ and ${\displaystyle b}$). Alternately, the vertices can be written as ${\displaystyle \{q,q+s,{}}$${\displaystyle q+s+is,q+is\}}$, in terms of a Gaussian integer ${\displaystyle q}$ representing the position of one vertex and another Gaussian integer ${\displaystyle s}$ representing a side.^{[ original research? ][ citation needed ]}

Compass and straightedge

The construction of a square with a given side, using a compass and straightedge, is given in Euclid's Elements I.46. ^[76] The existence of this construction means that squares are constructible polygons. A regular ${\displaystyle n}$-gon is constructible exactly when the odd prime factors of ${\displaystyle n}$ are distinct Fermat primes, ^[77] and in the case of a square ${\displaystyle n=4}$ has no odd prime factors so this condition is vacuously true.

Elements IV.6–7 also give constructions for a square inscribed in a circle and circumscribed about a circle, respectively. ^[78]

• 
  Square with a given circumcircle
• 
  Square with a given side length, using Thales' theorem
• 
  Square with a given diagonal

Related topics

The cube and regular octahedron, next steps in sequences of regular polytopes starting with squares

The Sierpiński carpet, a square fractal with square holes

An invariant measure for the baker's map

Because a square is a convex regular polygon with four sides, its Schläfli symbol is {4}. ^[79] A truncated square is an octagon. ^[80] The square is part of two infinite families of regular polytopes, one family which includes the cube in three dimensions and the hypercubes in higher dimensions, ^[81] and another which includes the regular octahedron in three dimensions and the cross polytopes in higher dimensions. ^[82] The cube and hypercubes can be given vertex coordinates that are all ${\displaystyle \pm 1}$, while the octahedron and cross polytopes can be given vertex coordinates that are ${\displaystyle \pm 1}$ in a single dimension and zero in all other dimensions. In two dimensions, the first system of coordinates produces an axis-parallel square, while the second system of coordinates produces a diagonal square. ^[83]

The Sierpiński carpet is a square fractal, with square holes. ^[84] Space-filling curves including the Hilbert curve, Peano curve, and Sierpiński curve cover a square as the continuous image of a line segment. ^[85] The Z-order curve is analogous but not continuous. ^[86] Other mathematical functions associated with squares include Arnold's cat map and the baker's map, which generate chaotic dynamical systems on a square, ^[87] and the lemniscate elliptic functions, complex functions periodic on a square grid. ^[88]

Inscribed squares

The Calabi triangle and the three placements of its largest square. The placement on the long side of the triangle is inscribed; the other two are not.

Main articles: Inscribed square problem and Inscribed square in a triangle

A square is said to be inscribed in a curve whenever all four vertices of the square lie on the curve. It is an unsolved problem in mathematics, the inscribed square problem, whether every simple closed curve has an inscribed square. Some cases of this problem are known to be true; for instance, it is true for every smooth curve. ^[90] For instance, a square can be inscribed on any circle, which becomes its circumscribed circle. As another special case of the inscribed square problem, a square can be inscribed on the boundary of any convex set. The only other regular polygon with this property is the equilateral triangle. More strongly, there exists a convex set on which no other regular polygon can be inscribed. ^[91]

For an inscribed square in a triangle, at least one of the square's sides lies on a side of the triangle. Every acute triangle has three inscribed squares, one lying on each of its three sides. In a right triangle there are two inscribed squares, one touching the right angle of the triangle and the other lying on the opposite side. An obtuse triangle has only one inscribed square, with a side coinciding with part of the triangle's longest side. An inscribed square can cover at most half the area of the triangle it is inscribed into. ^[92]

Area and quadrature

See also: Area, Quadrature (geometry), and Squaring the circle

The Pythagorean theorem: the two smaller squares on the sides of a right triangle have equal total area to the larger square on the hypotenuse.

A circle and square with the same area

Conventionally, since ancient times, most units of area have been defined in terms of various squares, typically a square with a standard unit of length as its side, for example a square meter or square inch. ^[93] The area of an arbitrary rectangle can then be simply computed as the product of its length and its width, and more complicated shapes can be measured by conceptually breaking them up into unit squares or into arbitrary triangles. ^[93]

In ancient Greek deductive geometry, the area of a planar shape was measured and compared by constructing a square with the same area by using only a finite number of steps with compass and straightedge, a process called quadrature or squaring. Euclid's Elements shows how to do this for rectangles, parallelograms, triangles, and then more generally for simple polygons by breaking them into triangular pieces. ^[94] Some shapes with curved sides could also be squared, such as the lune of Hippocrates ^[95] and the parabola. ^[96]

This use of a square as the defining shape for area measurement also occurs in the Greek formulation of the Pythagorean theorem: squares constructed on the two sides of a right triangle have equal total area to a square constructed on the hypotenuse. ^[97] Stated in this form, the theorem would be equally valid for other shapes on the sides of the triangle, such as equilateral triangles or semicircles, ^[98] but the Greeks used squares. In modern mathematics, this formulation of the theorem using areas of squares has been replaced by an algebraic formulation involving squaring numbers: the lengths of the sides and hypotenuse of the right triangle obey the equation ${\displaystyle a^{2}+b^{2}=c^{2}}$. ^[99]

Because of this focus on quadrature as a measure of area, the Greeks and later mathematicians sought unsuccessfully to square the circle, constructing a square with the same area as a given circle, again using finitely many steps with a compass and straightedge. In 1882, the task was proven to be impossible as a consequence of the Lindemann–Weierstrass theorem. This theorem proves that pi (π) is a transcendental number rather than an algebraic irrational number; that is, it is not the root of any polynomial with rational coefficients. A construction for squaring the circle could be translated into a polynomial formula for π, which does not exist. ^[100]

Tiling and packing

Main articles: Square tiling, Square packing, Circle packing in a square, and Squaring the square

Square tiling

Pythagorean tiling

The square tiling, familiar from flooring and game boards, is one of three regular tilings of the plane. The others are the tilings made from the equilateral triangle and the regular hexagon. ^[101] The vertices of the square tiling form a square lattice. ^[102] Squares of more than one size can also tile the plane, ^{[103] [104]} for instance in the Pythagorean tiling, named for its connection to proofs of the Pythagorean theorem. ^[105]

The smallest known square that can contain 11 unit squares has side length approximately 3.877084.

Square packing problems seek the smallest square or circle into which a given number of unit squares can fit. A chessboard optimally packs a square number of unit squares into a larger square, but beyond a few special cases such as this, the optimal solutions to these problems remain unsolved; ^{[106] [107] [108]} the same is true for circle packing in a square. ^[109] Packing squares into other shapes can have high computational complexity: testing whether a given number of unit squares can fit into an orthogonally convex rectilinear polygon with half-integer vertex coordinates is NP-complete. ^[110]

Squaring the square involves subdividing a given square into smaller squares, all having integer side lengths. A subdivision with distinct smaller squares is called a perfect squared square. ^[111] Another variant of squaring the square called "Mrs. Perkins's quilt" allows repetitions, but uses as few smaller squares as possible in order to make the greatest common divisor of the side lengths be 1. ^[112] The entire plane can be tiled by squares, with exactly one square of each integer side length. ^[113]

Counting

Main articles: Square pyramidal number and Dividing a square into similar rectangles

All 14 squares in a 3×3-square (4×4-vertex) grid

A common mathematical puzzle involves counting the squares of all sizes in a square grid of ${\displaystyle n\times n}$ squares. For instance, a square grid of nine squares has 14 squares: the nine squares that form the grid, four more ${\displaystyle 2\times 2}$ squares, and one ${\displaystyle 3\times 3}$ square. The answer to the puzzle is ${\displaystyle n(n+1)(2n+1)/6}$, a square pyramidal number. ^[114] For ${\displaystyle n=1,2,3,\dots }$ these numbers are: ^[115]

1, 5, 14, 30, 55, 91, 140, 204, 285, ...

A variant of the same puzzle asks for the number of squares formed by a grid of ${\displaystyle n\times n}$ points, allowing squares that are not axis-parallel. For instance, a grid of nine points has five axis-parallel squares as described above, but it also contains one more diagonal square for a total of six. ^[116] In this case, the answer is given by the 4-dimensional pyramidal numbers${\displaystyle n^{2}(n^{2}-1)/12}$. For ${\displaystyle n=1,2,3,\dots }$ these numbers are: ^[117]

0, 1, 6, 20, 50, 105, 196, 336, 540, ...

Three partitions of a square into similar rectangles

Another counting problem involving squares asks for the number of different shapes of rectangle that can be used when dividing a square into similar rectangles. ^[118] A square can be divided into two similar rectangles only in one way, by bisecting it, but when dividing a square into three similar rectangles there are three possible aspect ratios of the rectangles, 3:1, 3:2, and the square of the plastic ratio. The number of proportions that are possible when dividing into ${\displaystyle n}$ rectangles is known for small values of ${\displaystyle n}$, but not as a general formula. For ${\displaystyle n=1,2,3,\dots }$ these numbers are: ^[119]

1, 1, 3, 11, 51, 245, 1372, ...

Non-Euclidean geometry

Points (red) at equal distance from a central point (blue) according to taxicab geometry

Squares tilted at 45° to the coordinate axes are the metric balls for taxicab geometry, the ${\displaystyle L_{1}}$ distance metric in the real coordinate plane. According to this metric, the distance between any two points ${\displaystyle (x_{1},y_{1})}$ and ${\displaystyle (x_{2},y_{2})}$ is ${\displaystyle |x_{1}-x_{2}|+|y_{1}-y_{2}|}$ instead of the Euclidean distance ${\displaystyle {\sqrt {|x_{1}-x_{2}|^{2}+|y_{1}-y_{2}|^{2}}}}$. The points with taxicab distance ${\displaystyle d}$ from any given point form a diagonal square, centered at the given point, with diagonal length ${\displaystyle 2d}$. In the same way, axis-parallel squares form the metric balls for the ${\displaystyle L_{\infty }}$ distance metric (called the Chebyshev distance), for which the distance is given by the formula ${\displaystyle \max(|x_{1}-x_{2}|,|y_{1}-y_{2}|)}$. In this metric, the points with distance ${\displaystyle d}$ from some point form an axis-parallel square, centered at the given point, with side length ${\displaystyle 2d}$. ^{[120] [121] [122]}

Other forms of non-Euclidean geometry, including spherical geometry and hyperbolic geometry, also have polygons with four equal sides and equal angles. These are often called squares, ^[123] but some authors avoid calling them that, instead calling them regular quadrilaterals, because unlike Euclidean squares they cannot have right angles. These geometries also have regular polygons with right angles, but with numbers of sides different from four. ^[124]

In spherical geometry, a square is a polygon whose edges are great-circle arcs of equal length, which meet at equal angles. Unlike the square of Euclidean geometry, spherical squares have obtuse angles, larger than a right angle. Larger spherical squares have larger angles. ^[123] An octant of a sphere is a regular spherical triangle consisting of three straight sides and three right angles. The sphere can be tiled by eight such octants to make a spherical octahedron, with four octants meeting at each vertex. ^[125]

In hyperbolic geometry, squares with right angles do not exist. Rather, squares in hyperbolic geometry have acute angles, less than right angles. Larger hyperbolic squares have smaller angles. It is also possible to make a regular hyperbolic polygon with right angles at every vertex and any number of sides greater than four; such polygons can uniformly tile the hyperbolic plane. ^[124]

• 
  Subdividing a sphere into hemispheres forms a dihedron with spherical square faces and 180° angles. The Peirce quincuncial projection for world maps conformally maps these faces to Euclidean squares. ^[126]
• 
  Six squares can tile the sphere with 3 squares around each vertex and 120° internal angles. This is called a spherical cube. ^[127]
• 
  Squares can tile the hyperbolic plane with five around each vertex, each square having 72° internal angles, giving the order-5 square tiling. For any n ≥ 5 there is a hyperbolic tiling with n squares about each vertex. ^[124]
• 
  Regular hexagons with right angles can tile the hyperbolic plane with four hexagons meeting at each vertex, giving the order-4 hexagonal tiling. For any n ≥ 5 there is a hyperbolic tiling by right-angled regular n-gons, dual to the tiling with n squares about each vertex. ^[124]

See also

• Mathematics portal

• Clifford torus, 4d embedding of a square glued edge-to-edge into a torus
• Finsler–Hadwiger theorem on a square derived from two squares sharing a vertex
• Midsquare quadrilateral, a polygon whose edge midpoints form a square
• Monsky's theorem, on subdividing a square into an odd number of equal-area triangles
• Paper bag problem, on the volume enclosed by two squares glued edge to edge
• Square trisection, a problem of cutting and reassembling one square into three squares
• Squircle, a shape intermediate between a square and a circle
• Tarski's circle-squaring problem, dividing a disk into sets that can be rearranged into a square
• Van Aubel's theorem and Thébault's theorem, on squares placed on the sides of a quadrilateral

References

1. Usiskin, Zalman; Griffin, Jennifer (2008). The Classification of Quadrilaterals: A Study of Definition. Information Age Publishing. p. 59. ISBN   1-59311-695-0.
2. Wilson, Jim (Summer 2010). "Problem Set 1.3, problem 10". Math 5200/7200 Foundations of Geometry I. University of Georgia. Retrieved 2025-02-05.
3. Alsina, Claudi; Nelsen, Roger B. (2020). "Theorem 9.2.1". A Cornucopia of Quadrilaterals. Dolciani Mathematical Expositions. Vol. 55. American Mathematical Society. p. 186. ISBN   9781470453121.
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v t e Fundamental convex regular and uniform polytopes in dimensions 2–10
Family	An	Bn	I2(p) / Dn	E6 / E7 / E8 / F4 / G2	Hn
Regular polygon	Triangle	Square	p-gon	Hexagon	Pentagon
Uniform polyhedron	Tetrahedron	Octahedron • Cube	Demicube		Dodecahedron • Icosahedron
Uniform polychoron	Pentachoron	16-cell • Tesseract	Demitesseract	24-cell	120-cell • 600-cell
Uniform 5-polytope	5-simplex	5-orthoplex • 5-cube	5-demicube
Uniform 6-polytope	6-simplex	6-orthoplex • 6-cube	6-demicube	122 • 221
Uniform 7-polytope	7-simplex	7-orthoplex • 7-cube	7-demicube	132 • 231 • 321
Uniform 8-polytope	8-simplex	8-orthoplex • 8-cube	8-demicube	142 • 241 • 421
Uniform 9-polytope	9-simplex	9-orthoplex • 9-cube	9-demicube
Uniform 10-polytope	10-simplex	10-orthoplex • 10-cube	10-demicube
Uniform n-polytope	n-simplex	n-orthoplex • n-cube	n-demicube	1k2 • 2k1 • k21	n-pentagonal polytope
Topics: Polytope families • Regular polytope • List of regular polytopes and compounds