Representations Line segments in the golden ratio 1.618033988749894...[1] ${\displaystyle {\frac {1+{\sqrt {5))}{2))}$ ${\displaystyle 1+{\cfrac {1}{1+{\cfrac {1}{1+{\cfrac {1}{1+{\cfrac {1}{1+\ddots ))))))))}$ 1.10011110001101110111... 1.9E3779B97F4A7C15...

In mathematics, two quantities are in the golden ratio if their ratio is the same as the ratio of their sum to the larger of the two quantities. Expressed algebraically, for quantities ${\displaystyle a}$ and ${\displaystyle b}$ with ${\displaystyle a>b>0,}$

${\displaystyle {\frac {a+b}{a))={\frac {a}{b))\ {=:}\ \varphi }$

where the Greek letter phi (${\displaystyle \varphi }$ or ${\displaystyle \phi }$) represents the golden ratio.[a] It is an irrational number that is a solution to the quadratic equation ${\displaystyle x^{2}-x-1=0,}$ with a value of[2][1]

${\displaystyle \varphi ={\frac {1+{\sqrt {5))}{2))=}$1.618033988749....()

The golden ratio is also called the golden mean or golden section (Latin: sectio aurea).[3][4] Other names include extreme and mean ratio,[5] medial section, divine proportion (Latin: proportio divina),[6] divine section (Latin: sectio divina), golden proportion, golden cut,[7] and golden number.[8][9][10]

Mathematicians since Euclid have studied the properties of the golden ratio, including its appearance in the dimensions of a regular pentagon and in a golden rectangle, which may be cut into a square and a smaller rectangle with the same aspect ratio. The golden ratio has also been used to analyze the proportions of natural objects as well as man-made systems such as financial markets, in some cases based on dubious fits to data.[11] The golden ratio appears in some patterns in nature, including the spiral arrangement of leaves and other parts of vegetation.

Some 20th-century artists and architects, including Le Corbusier and Salvador Dalí, have proportioned their works to approximate the golden ratio, believing this to be aesthetically pleasing. These often appear in the form of the golden rectangle, in which the ratio of the longer side to the shorter is the golden ratio.

## Calculation

Two quantities ${\displaystyle a}$ and ${\displaystyle b}$ are said to be in the golden ratio ${\displaystyle \varphi }$ if

${\displaystyle {\frac {a+b}{a))={\frac {a}{b))=\varphi .}$

One method for finding the value of ${\displaystyle \varphi }$ is to start with the left fraction. Through simplifying the fraction and substituting in ${\displaystyle b/a=1/\varphi ,}$

${\displaystyle {\frac {a+b}{a))={\frac {a}{a))+{\frac {b}{a))=1+{\frac {b}{a))=1+{\frac {1}{\varphi )).}$

Therefore,

${\displaystyle 1+{\frac {1}{\varphi ))=\varphi .}$

Multiplying by ${\displaystyle \varphi }$ gives

${\displaystyle \varphi +1=\varphi ^{2))$

which can be rearranged to

${\displaystyle {\varphi }^{2}-\varphi -1=0.}$

Using the quadratic formula, two solutions are obtained:

${\displaystyle {\frac {1+{\sqrt {5))}{2))=1.618033\dots }$ and ${\displaystyle {\frac {1-{\sqrt {5))}{2))=-0.618033\dots .}$

Because ${\displaystyle \varphi }$ is the ratio between positive quantities, ${\displaystyle \varphi }$ is necessarily the positive one. However, the negative root, ${\displaystyle -{\frac {1}{\varphi ))}$, shares many properties with the golden ratio.

## History

According to Mario Livio,

Some of the greatest mathematical minds of all ages, from Pythagoras and Euclid in ancient Greece, through the medieval Italian mathematician Leonardo of Pisa and the Renaissance astronomer Johannes Kepler, to present-day scientific figures such as Oxford physicist Roger Penrose, have spent endless hours over this simple ratio and its properties. ... Biologists, artists, musicians, historians, architects, psychologists, and even mystics have pondered and debated the basis of its ubiquity and appeal. In fact, it is probably fair to say that the Golden Ratio has inspired thinkers of all disciplines like no other number in the history of mathematics.[13]

— The Golden Ratio: The Story of Phi, the World's Most Astonishing Number

Ancient Greek mathematicians first studied the golden ratio because of its frequent appearance in geometry;[14] the division of a line into "extreme and mean ratio" (the golden section) is important in the geometry of regular pentagrams and pentagons.[15] According to one story, 5th-century BC mathematician Hippasus discovered that the golden ratio was neither a whole number nor a fraction (an irrational number), surprising Pythagoreans.[16] Euclid's Elements (c. 300 BC) provides several propositions and their proofs employing the golden ratio,[17][b] and contains its first known definition which proceeds as follows:[18]

A straight line is said to have been cut in extreme and mean ratio when, as the whole line is to the greater segment, so is the greater to the lesser.[19][c]

The golden ratio was studied peripherally over the next millennium. Abu Kamil (c. 850–930) employed it in his geometric calculations of pentagons and decagons; his writings influenced that of Fibonacci (Leonardo of Pisa) (c. 1170–1250), who used the ratio in related geometry problems although he never connected it to the series of numbers named after him.[21]

Luca Pacioli named his book Divina proportione (1509) after the ratio, and explored its properties including its appearance in some of the Platonic solids.[10][22] Leonardo da Vinci, who illustrated the aforementioned book, called the ratio the sectio aurea ('golden section').[23] 16th-century mathematicians such as Rafael Bombelli solved geometric problems using the ratio.[24]

German mathematician Simon Jacob (d. 1564) noted that consecutive Fibonacci numbers converge to the golden ratio;[25] this was rediscovered by Johannes Kepler in 1608.[26] The first known decimal approximation of the (inverse) golden ratio was stated as "about ${\displaystyle 0.6180340}$" in 1597 by Michael Maestlin of the University of Tübingen in a letter to Kepler, his former student.[27] The same year, Kepler wrote to Maestlin of the Kepler triangle, which combines the golden ratio with the Pythagorean theorem. Kepler said of these:

Geometry has two great treasures: one is the theorem of Pythagoras, the other the division of a line into extreme and mean ratio. The first we may compare to a mass of gold, the second we may call a precious jewel.[6]

18th-century mathematicians Abraham de Moivre, Daniel Bernoulli, and Leonhard Euler used a golden ratio-based formula which finds the value of a Fibonacci number based on its placement in the sequence; in 1843, this was rediscovered by Jacques Philippe Marie Binet, for whom it was named "Binet's formula".[28] Martin Ohm first used the German term goldener Schnitt ('golden section') to describe the ratio in 1835.[29] James Sully used the equivalent English term in 1875.[30]

By 1910, mathematician Mark Barr began using the Greek letter Phi (${\displaystyle {\boldsymbol {\varphi ))}$) as a symbol for the golden ratio.[31][d] It has also been represented by tau (${\displaystyle {\boldsymbol {\tau ))}$), the first letter of the ancient Greek τομή ('cut' or 'section').[34][35]

The zome construction system, developed by Steve Baer in the late 1960s, is based on the symmetry system of the icosahedron/dodecahedron, and uses the golden ratio ubiquitously. Between 1973 and 1974, Roger Penrose developed Penrose tiling, a pattern related to the golden ratio both in the ratio of areas of its two rhombic tiles and in their relative frequency within the pattern.[36] This led to Dan Shechtman's early 1980s discovery of quasicrystals,[37][38] some of which exhibit icosahedral symmetry.[39][40]

## Mathematics

### Irrationality

The golden ratio is an irrational number. Below are two short proofs of irrationality:

#### Contradiction from an expression in lowest terms

Recall that:

the whole is the longer part plus the shorter part;
the whole is to the longer part as the longer part is to the shorter part.

If we call the whole ${\displaystyle n}$ and the longer part ${\displaystyle m,}$ then the second statement above becomes

${\displaystyle n}$ is to ${\displaystyle m}$ as ${\displaystyle m}$ is to ${\displaystyle n-m.}$

To say that the golden ratio ${\displaystyle \varphi }$ is rational means that ${\displaystyle \varphi }$ is a fraction ${\displaystyle n/m}$ where ${\displaystyle n}$ and ${\displaystyle m}$ are integers. We may take ${\displaystyle n/m}$ to be in lowest terms and ${\displaystyle n}$ and ${\displaystyle m}$ to be positive. But if ${\displaystyle n/m}$ is in lowest terms, then the equally valued ${\displaystyle m/(n-m)}$ is in still lower terms. That is a contradiction that follows from the assumption that ${\displaystyle \varphi }$ is rational.

#### By irrationality of ${\displaystyle {\sqrt {5))}$

Another short proof – perhaps more commonly known – of the irrationality of the golden ratio makes use of the closure of rational numbers under addition and multiplication. If ${\displaystyle \varphi ={\tfrac {1}{2))(1+{\sqrt {5)))}$ is rational, then ${\displaystyle 2\varphi -1={\sqrt {5))}$ is also rational, which is a contradiction if it is already known that the square root of a non-square natural number is irrational.

### Minimal polynomial

The golden ratio is also an algebraic number and even an algebraic integer. It has minimal polynomial

${\displaystyle x^{2}-x-1.}$

This quadratic polynomial has two roots, ${\displaystyle \varphi }$ and ${\displaystyle -\varphi ^{-1}.}$

The golden ratio is also closely related to the polynomial

${\displaystyle x^{2}+x-1,}$

which has roots ${\displaystyle -\varphi }$ and ${\displaystyle \varphi ^{-1}.}$

### Golden ratio conjugate and powers

The conjugate root to the minimal polynomial ${\displaystyle x^{2}-x-1}$ is

${\displaystyle -{\frac {1}{\varphi ))=1-\varphi ={\frac {1-{\sqrt {5))}{2))=-0.618033\dots .}$

The absolute value of this quantity (${\displaystyle 0.618\ldots }$) corresponds to the length ratio taken in reverse order (shorter segment length over longer segment length, ${\displaystyle b/a}$), and is sometimes referred to as the golden ratio conjugate[12] or silver ratio.[e][41] It is denoted here by the capital Phi (${\displaystyle {\boldsymbol {\Phi ))}$):

${\displaystyle \Phi ={\frac {1}{\varphi ))=\varphi ^{-1}=\varphi -1=0.618033\ldots .}$

This illustrates the unique property of the golden ratio among positive numbers, that

${\displaystyle {\frac {1}{\varphi ))=\varphi -1,}$

or its inverse:

${\displaystyle {\frac {1}{\Phi ))=\Phi +1.}$

The conjugate and the defining quadratic polynomial relationship lead to decimal values that have their fractional part in common with ${\displaystyle \varphi }$:

{\displaystyle {\begin{aligned}\varphi ^{2}&=\varphi +1=2.618033\dots ,\\[5mu]{\frac {1}{\varphi ))&=\varphi -1=0.618033\dots .\end{aligned))}

The sequence of powers of ${\displaystyle \varphi }$ contains these values ${\displaystyle 0.618033\ldots ,}$ ${\displaystyle 1.0,}$ ${\displaystyle 1.618033\ldots ,}$ ${\displaystyle 2.618033\ldots ;}$ more generally, any power of ${\displaystyle \varphi }$ is equal to the sum of the two immediately preceding powers:

${\displaystyle \varphi ^{n}=\varphi ^{n-1}+\varphi ^{n-2}=\varphi \cdot \operatorname {F} _{n}+\operatorname {F} _{n-1}.}$

As a result, one can easily decompose any power of ${\displaystyle \varphi }$ into a multiple of ${\displaystyle \varphi }$ and a constant. The multiple and the constant are always adjacent Fibonacci numbers. This leads to another property of the positive powers of ${\displaystyle \varphi }$:

If ${\displaystyle \lfloor n/2-1\rfloor =m,}$ then:

{\displaystyle {\begin{aligned}\varphi ^{n}&=\varphi ^{n-1}+\varphi ^{n-3}+\cdots +\varphi ^{n-1-2m}+\varphi ^{n-2-2m}\\[5mu]\varphi ^{n}-\varphi ^{n-1}&=\varphi ^{n-2}.\end{aligned))}

### Alternative forms

The formula ${\displaystyle \varphi =1+1/\varphi }$ can be expanded recursively to obtain a continued fraction for the golden ratio:[42]

${\displaystyle \varphi =[1;1,1,1,\dots ]=1+{\cfrac {1}{1+{\cfrac {1}{1+{\cfrac {1}{1+\ddots ))))))}$

and its reciprocal:

${\displaystyle \varphi ^{-1}=[0;1,1,1,\dots ]=0+{\cfrac {1}{1+{\cfrac {1}{1+{\cfrac {1}{1+\ddots ))))))}$

The convergents of these continued fractions (${\displaystyle 1/1,}$ ${\displaystyle 2/1,}$ ${\displaystyle 2/1,}$ ${\displaystyle 3/2,}$ ${\displaystyle 5/3,}$ ${\displaystyle 8/5,}$ ${\displaystyle 13/8,}$ ... or ${\displaystyle 1/1,}$ ${\displaystyle 1/2,}$ ${\displaystyle 2/3,}$ ${\displaystyle 3/5,}$ ${\displaystyle 5/8,}$ ${\displaystyle 8/13,}$ ...) are ratios of successive Fibonacci numbers.

The equation ${\displaystyle \varphi ^{2}=1+\varphi }$ likewise produces the continued square root:

${\displaystyle \varphi ={\sqrt {1+{\sqrt {1+{\sqrt {1+\cdots )))))).}$

An infinite series can be derived to express ${\displaystyle \varphi }$:[43]

${\displaystyle \varphi ={\frac {13}{8))+\sum _{n=0}^{\infty }{\frac {(-1)^{n+1}(2n+1)!}{4^{2n+3}n!(n+2)!)).}$

Also:

{\displaystyle {\begin{aligned}\varphi &=1+2\sin(\pi /10)=1+2\sin 18^{\circ },\\[5mu]\varphi &={\tfrac {1}{2))\csc(\pi /10)={\tfrac {1}{2))\csc 18^{\circ },\\[5mu]\varphi &=2\cos(\pi /5)=2\cos 36^{\circ },\\[5mu]\varphi &=2\sin(3\pi /10)=2\sin 54^{\circ }.\end{aligned))}

These correspond to the fact that the length of the diagonal of a regular pentagon is ${\displaystyle \varphi }$ times the length of its side, and similar relations in a pentagram.

#### Decimal expansion

The golden ratio's decimal expansion can be calculated from the expression

${\displaystyle \varphi ={1+{\sqrt {5)) \over 2))$

with ${\displaystyle {\sqrt {5))=}$ 2.236067977.... . The square root of ${\displaystyle 5}$ can be calculated via the Babylonian method, starting with an initial estimate such as ${\displaystyle x_{0}=2}$ and iterating

${\displaystyle x_{n+1}={\frac {1}{2))\left(x_{n}+{\frac {5}{x_{n))}\right)}$

for ${\displaystyle n=0,1,2,3,\ldots ,}$ until the difference between ${\displaystyle x_{n))$ and ${\displaystyle x_{n-1))$ becomes zero to the desired number of digits, to yield

${\displaystyle \varphi \approx {\tfrac {1}{2))(1+x_{n}).}$

The Babylonian algorithm for ${\displaystyle {\sqrt {5))}$ is equivalent to Newton's method for solving the equation ${\displaystyle x^{2}-5=0,}$ and it converges quadratically, meaning that the number of correct digits is roughly doubled each iteration. To avoid the computationally expensive division operation, Newton's method can instead be used to solve the equation ${\displaystyle 5x^{-2}-4=0}$ for the root ${\textstyle {\tfrac {1}{2)){\sqrt {5)).}$ Then,

${\textstyle \varphi \approx {\tfrac {1}{2))+x_{n},}$

and the update step is

${\displaystyle x_{n+1}={\tfrac {3}{2))x_{n}-{\tfrac {2}{5))x_{n}^{3}.}$

Alternately, Newton's method can be applied directly to any equation that has the golden ratio as a solution, such as ${\displaystyle x^{2}-x-1=0.}$ In this case,

${\textstyle \varphi \approx x_{n},}$

with the update step

${\displaystyle x_{n+1}={\frac {x_{n}^{2}+1}{2x_{n}-1))\,.}$

Halley's method has cubic convergence (roughly tripling the number of correct digits with each iteration), but may be slower for practical computation because each step takes more work. To solve ${\displaystyle x^{2}-x-1=0,}$ the update step is

${\displaystyle x_{n+1}={\frac {x_{n}^{3}+3x_{n}-1}{3x_{n}^{2}-3x_{n}+2))\,.}$

### Geometry

#### Line segment

##### Dividing by interior division
1. Having a line segment ${\displaystyle AB,}$ construct a perpendicular ${\displaystyle BC}$ at point ${\displaystyle B,}$ with ${\displaystyle BC}$ half the length of ${\displaystyle AB.}$ Draw the hypotenuse ${\displaystyle AC.}$
2. Draw an arc with center ${\displaystyle C}$ and radius ${\displaystyle BC.}$ This arc intersects the hypotenuse ${\displaystyle AC}$ at point ${\displaystyle D.}$
3. Draw an arc with center ${\displaystyle A}$ and radius ${\displaystyle AD.}$ This arc intersects the original line segment ${\displaystyle AB}$ at point ${\displaystyle S.}$ Point ${\displaystyle S}$ divides the original line segment ${\displaystyle AB}$ into line segments ${\displaystyle AS}$ and ${\displaystyle SB}$ with lengths in the golden ratio.
##### Dividing by exterior division
1. Draw a line segment ${\displaystyle AS}$ and construct off the point ${\displaystyle S}$ a segment ${\displaystyle SC}$ perpendicular to ${\displaystyle AS}$ and with the same length as ${\displaystyle AS.}$
2. Do bisect the line segment ${\displaystyle AS}$ with ${\displaystyle M.}$
3. A circular arc around ${\displaystyle M}$ with radius ${\displaystyle MC}$ intersects in point ${\displaystyle B}$ the straight line through points ${\displaystyle A}$ and ${\displaystyle S}$ (also known as the extension of ${\displaystyle AS}$). The ratio of ${\displaystyle AS}$ to the constructed segment ${\displaystyle SB}$ is the golden ratio.

Application examples you can see in the articles Pentagon with a given side length, Decagon with given circumcircle and Decagon with a given side length.

Both of the above displayed different algorithms produce geometric constructions that determine two aligned line segments where the ratio of the longer one to the shorter one is the golden ratio.

##### Odom's construction

George Odom has given a remarkably simple construction for ${\displaystyle \varphi }$ involving an equilateral triangle: if an equilateral triangle is inscribed in a circle and the line segment joining the midpoints of two sides is produced to intersect the circle in either of two points, then these three points are in golden proportion. This result is a straightforward consequence of the intersecting chords theorem and can be used to construct a regular pentagon, a construction that attracted the attention of the noted Canadian geometer H. S. M. Coxeter who published it in Odom's name as a diagram in the American Mathematical Monthly accompanied by the single word "Behold!" [44]

##### Golden triangle

The golden triangle can be characterized as an isosceles triangle ${\displaystyle ABC}$ with the property that bisecting the angle ${\displaystyle C}$ produces a new triangle ${\displaystyle CXB}$ which is a similar triangle to the original.

If angle ${\displaystyle BCX=\alpha ,}$ then ${\displaystyle XCA=\alpha }$ because of the bisection, and ${\displaystyle CAB=\alpha }$ because of the similar triangles; ${\displaystyle ABC=2\alpha }$ from the original isosceles symmetry, and ${\displaystyle BXC=2\alpha }$ by similarity. The angles in a triangle add up to ${\displaystyle 180^{\circ },}$ so ${\displaystyle 5\alpha =180^{\circ },}$ giving ${\displaystyle \alpha =36^{\circ }.}$ So the angles of the golden triangle are thus ${\displaystyle 36^{\circ ))$${\displaystyle 72^{\circ ))$${\displaystyle 72^{\circ }.}$ The angles of the remaining obtuse isosceles triangle ${\displaystyle AXC}$ (sometimes called the golden gnomon) are ${\displaystyle 36^{\circ ))$${\displaystyle 36^{\circ ))$${\displaystyle 108^{\circ }.}$

Suppose ${\displaystyle XB}$ has length ${\displaystyle 1,}$ and we call ${\displaystyle BC}$ length ${\displaystyle \varphi .}$ Because of the isosceles triangles ${\displaystyle XC=XA}$ and ${\displaystyle BC=XC,}$ so these are also length ${\displaystyle \varphi .}$ Length ${\displaystyle AC=AB,}$ therefore equals ${\displaystyle \varphi +1.}$ But triangle ${\displaystyle ABC}$ is similar to triangle ${\displaystyle CXB,}$ so ${\displaystyle AC/BC=BC/BX,}$ ${\displaystyle AC/\varphi =\varphi /1,}$ and so ${\displaystyle AC}$ also equals ${\displaystyle \varphi ^{2}.}$ Thus ${\displaystyle \varphi ^{2}=\varphi +1,}$ confirming that ${\displaystyle \varphi }$ is indeed the golden ratio.

Similarly, the ratio of the area of the larger triangle ${\displaystyle AXC}$ to the smaller ${\displaystyle CXB}$ is equal to ${\displaystyle \varphi ,}$ while the inverse ratio is ${\displaystyle \varphi -1.}$

##### Golden rectangle

The golden ratio proportions the adjacent side lengths of a golden rectangle in ${\displaystyle 1:\varphi }$ ratio.[45] They are special in that stacking them produces golden rectangles as well. Pairs of opposing vertices in an icosahedron form a golden rectange.[46] Three golden rectangles inside an icosahedron intersect each other at 90° degree angles, and collectively contain 12 of its vertices.

##### Golden rhombus

A golden rhombus is a rhombus whose diagonals are in proportion to the golden ratio. The rhombic triacontahedron is a Catalan solid with golden rhombi as faces that contain diagonals in ${\displaystyle 1:\varphi }$ ratio. The dihedral angle between any two adjacent rhombi in a rhombic triacontahedron is ${\displaystyle 144^{\circ },}$ which is twice the isosceles angle of a golden triangle and four times its most acute angle.[47] A golden rhombus whose diagonals are in ratio of ${\displaystyle 1:\varphi ^{2))$ is present in the rhombic enneacontahedron, a zonohedron with resemblance to the rhombic triacontahedron. These rhombi have angles appriximating 70.528° and 109.471° degrees, and make up 30 of 60 rhombic faces in the polyhedron.

##### Golden pyramid

A pyramid in which the apothem (slant height along the bisector of a face) is equal to ${\displaystyle \varphi }$ times the semi-base (half the base width) is sometimes called a golden pyramid. The isosceles triangle that is the face of such a pyramid can be constructed from the two halves of a diagonally split golden rectangle (of size semi-base by apothem), joining the medium-length edges to make the apothem. The height of this pyramid is ${\displaystyle {\sqrt {\varphi ))}$ times the semi-base (that is, the slope of the face is ${\displaystyle {\sqrt {\varphi ))}$); the square of the height is equal to the area of a face, ${\displaystyle \varphi }$ times the square of the semi-base. The medial right triangle of this "golden" pyramid (see diagram), with sides ${\displaystyle 1\mathbin {:} {\sqrt {\varphi ))\mathbin {:} \varphi }$ is interesting in its own right, demonstrating via the Pythagorean theorem the relationship ${\textstyle {\sqrt {\varphi ))={\sqrt {\varphi ^{2}-1))}$ or ${\displaystyle \varphi ={\sqrt {1+\varphi )).}$ This Kepler triangle[48] is the only right triangle proportion with edge lengths in geometric progression,[49][50] just as the ${\displaystyle 3\mathbin {:} 4\mathbin {:} 5}$ triangle is the only right triangle proportion with edge lengths in arithmetic progression. The angle with tangent ${\displaystyle {\sqrt {\varphi ))}$ corresponds to the angle that the side of the pyramid makes with respect to the ground, ${\displaystyle 51.827^{\circ ))$ (${\displaystyle 51^{\circ }\;49'\;38''}$).[51]

#### Pentagonal

##### Pentagon

In a regular pentagon the ratio of a diagonal to a side is the golden ratio, while intersecting diagonals section each other in the golden ratio.[10] The golden ratio properties of a regular pentagon can be confirmed by applying Ptolemy's theorem to the quadrilateral formed by removing one of its vertices. If the quadrilateral's long edge and diagonals are ${\displaystyle b,}$ and short edges are ${\displaystyle a,}$ then Ptolemy's theorem gives ${\displaystyle b^{2}=a^{2}+ab}$ which yields

${\displaystyle {b \over a}=((1+{\sqrt {5))} \over 2}.}$
##### Pentagram

The golden ratio plays an important role in the geometry of pentagrams. Each intersection of edges sections other edges in the golden ratio. Also, the ratio of the length of the shorter segment to the segment bounded by the two intersecting edges (a side of the pentagon in the pentagram's center) is ${\displaystyle \varphi ,}$ as the four-color illustration shows.

The pentagram includes ten isosceles triangles: five acute and five obtuse isosceles triangles. In all of them, the ratio of the longer side to the shorter side is ${\displaystyle \varphi .}$ The acute triangles are golden triangles. The obtuse isosceles triangles are golden gnomons.

### Relationship to Fibonacci sequence

The mathematics of the golden ratio and of the Fibonacci sequence are intimately interconnected. The Fibonacci sequence is:

${\displaystyle 1,}$ ${\displaystyle 1,}$ ${\displaystyle 2,}$ ${\displaystyle 3,}$ ${\displaystyle 5,}$ ${\displaystyle 8,}$ ${\displaystyle 13,}$ ${\displaystyle 21,}$ ${\displaystyle 34,}$ ${\displaystyle 55,}$ ${\displaystyle 89,}$ ${\displaystyle 144,}$ ${\displaystyle 233,}$ ${\displaystyle 377,}$ ${\displaystyle 610,}$ ${\displaystyle 987,}$ ...

A closed-form expression for the Fibonacci sequence involves the golden ratio:

${\displaystyle F\left(n\right)=((\varphi ^{n}-(1-\varphi )^{n)) \over {\sqrt {5))}=((\varphi ^{n}-(-\varphi )^{-n)) \over {\sqrt {5))}.}$

The golden ratio is the limit of the ratios of successive terms of the Fibonacci sequence (or any Fibonacci-like sequence), as shown by Kepler:[52]

${\displaystyle \lim _{n\to \infty }{\frac {F_{n+1)){F_{n))}=\varphi .}$

In other words, if a Fibonacci number is divided by its immediate predecessor in the sequence, the quotient approximates ${\displaystyle \varphi ;}$ e.g., ${\displaystyle 987/610\approx 1.6180327868852.}$ These approximations are alternately lower and higher than ${\displaystyle \varphi ,}$ and converge to ${\displaystyle \varphi }$ as the Fibonacci numbers increase, and:

${\displaystyle \sum _{n=1}^{\infty }|F_{n}\varphi -F_{n+1}|=\varphi .}$

More generally

${\displaystyle \lim _{n\to \infty }{\frac {F_{n+a)){F_{n))}=\varphi ^{a},}$

where above, the ratios of consecutive terms of the Fibonacci sequence, is a case when ${\displaystyle a=1.}$

Furthermore, the successive powers of ${\displaystyle \varphi }$ obey the Fibonacci recurrence

${\displaystyle \varphi ^{n+1}=\varphi ^{n}+\varphi ^{n-1}.}$

This identity allows any polynomial in ${\displaystyle \varphi }$ to be reduced to a linear expression. For example:

{\displaystyle {\begin{aligned}3\varphi ^{3}-5\varphi ^{2}+4&=3(\varphi ^{2}+\varphi )-5\varphi ^{2}+4\\&=3[(\varphi +1)+\varphi ]-5(\varphi +1)+4\\&=\varphi +2\approx 3.618033.\end{aligned))}

The reduction to a linear expression can be accomplished in one step by using the relationship

${\displaystyle \varphi ^{k}=F_{k}\varphi +F_{k-1},}$

where ${\displaystyle F_{k))$ is the ${\displaystyle k}$th Fibonacci number.

However, this is no special property of ${\displaystyle \varphi ,}$ because polynomials in any solution ${\displaystyle x}$ to a quadratic equation can be reduced in an analogous manner, by applying:

${\displaystyle x^{2}=ax+b}$

for given coefficients ${\displaystyle a,}$ ${\displaystyle b}$ such that ${\displaystyle x}$ satisfies the equation. Even more generally, any rational function (with rational coefficients) of the root of an irreducible ${\displaystyle n}$th-degree polynomial over the rationals can be reduced to a polynomial of degree ${\displaystyle n-1.}$ Phrased in terms of field theory, if ${\displaystyle \alpha }$ is a root of an irreducible ${\displaystyle n}$th-degree polynomial, then ${\displaystyle \mathbb {Q} (\alpha )}$ has degree ${\displaystyle n}$ over ${\displaystyle \mathbb {Q} ,}$ with basis ${\displaystyle \{1,\alpha ,\ldots ,\alpha ^{n-1}\}.}$

### Other properties

It is relatively easy to compute the decimal expansion of the golden ratio with arbitrary precision, due to the simplicity in the equations mentioned above. The time needed to compute ${\displaystyle n}$ digits of the golden ratio is proportional to the time needed to divide two ${\displaystyle n}$-digit numbers. This is considerably faster than known algorithms for the transcendental numbers ${\displaystyle \pi }$ and ${\displaystyle e}$. An easily programmed alternative using only integer arithmetic is to calculate two large consecutive Fibonacci numbers and divide them. The ratio of Fibonacci numbers ${\displaystyle F_{25001))$ and ${\displaystyle F_{25000},}$ each over ${\displaystyle 5000}$ digits, yields over ${\displaystyle 10{,}000}$ significant digits of the golden ratio. The decimal expansion of the golden ratio ${\displaystyle \varphi }$[1] has been calculated to an accuracy of ten trillion (${\displaystyle 1\times 10^{13}=10{,}000{,}000{,}000{,}000}$) digits.[53]

The golden ratio has the simplest expression (and slowest convergence) as a continued fraction expansion of any irrational number (see Alternate forms above). It is, for that reason, one of the worst cases of Lagrange's approximation theorem and it is an extremal case of the Hurwitz inequality for Diophantine approximations. This may be why angles close to the golden ratio often appear in phyllotaxis.[54]

The golden ratio and inverse golden ratio ${\displaystyle \varphi _{\pm }={\tfrac {1}{2)){\bigl (}1\pm {\sqrt {5)){\bigr )))$ have a set of symmetries that preserve and interrelate them. They are both preserved by the fractional linear transformations ${\displaystyle x,1/(1-x),(x-1)/x}$ – this fact corresponds to the identity and the definition quadratic equation. Further, they are interchanged by the three maps ${\displaystyle 1/x,1-x,x/(x-1)}$ – they are reciprocals, symmetric about ${\displaystyle {\tfrac {1}{2)),}$ and (projectively) symmetric about ${\displaystyle 2.}$ More deeply, these maps form a subgroup of the modular group ${\displaystyle \operatorname {PSL} (2,\mathbb {Z} )}$ isomorphic to the symmetric group on ${\displaystyle 3}$ letters, ${\displaystyle S_{3},}$ corresponding to the stabilizer of the set ${\displaystyle \{0,1,\infty \))$ of ${\displaystyle 3}$ standard points on the projective line, and the symmetries correspond to the quotient map ${\displaystyle S_{3}\to S_{2))$ – the subgroup ${\displaystyle C_{3} consisting of the identity and the ${\displaystyle 3}$-cycles, in cycle notation ${\displaystyle \{(1),(0\,1\,\infty ),(0\,\infty \,1)\},}$ fixes the two numbers, while the ${\displaystyle 2}$-cycles ${\displaystyle \{(0\,1),(0\,\infty ),(1\,\infty )\))$ interchange these, thus realizing the map.

The golden ratio is a fundamental unit of the algebraic number field ${\displaystyle \mathbb {Q} ({\sqrt {5)))}$ and is a Pisot–Vijayaraghavan number.[55] In the field ${\displaystyle \mathbb {Q} ({\sqrt {5)))}$ we have ${\displaystyle \varphi ^{n}={\tfrac {1}{2)){\bigl (}L_{n}+F_{n}{\sqrt {5)){\bigr )},}$ where ${\displaystyle L_{n))$ is the ${\displaystyle n}$-th Lucas number.

For the gamma function,[f] the only solutions to the equation Γ(z − 1) = Γ(z + 1) are z = φ and z = −1/φ.

When the golden ratio is used as the base of a numeral system (see golden ratio base, sometimes dubbed phinary or ${\displaystyle \varphi }$-nary), quadratic integers in the ring ${\displaystyle \mathbb {Z} [\varphi ]}$ – that is, numbers of the form ${\displaystyle a+b\varphi }$ for ${\displaystyle a,b\in \mathbb {Z} }$ – have terminating representations, but rational fractions have non-terminating representations.

The golden ratio appears in the theory of modular functions as well. For ${\displaystyle \left|q\right|<1}$, let

${\displaystyle R(q)={\cfrac {q^{1/5)){1+{\cfrac {q}{1+{\cfrac {q^{2)){1+{\cfrac {q^{3)){1+\ddots )))))))).}$

Then

${\displaystyle R(e^{-2\pi })={\sqrt {\varphi {\sqrt {5))))-\varphi ,\quad R(-e^{-\pi })=\varphi ^{-1}-{\sqrt {2-\varphi ^{-1))))$

and

${\displaystyle R(e^{-2\pi i/\tau })={\frac {1-\varphi R(e^{2\pi i\tau })}{\varphi +R(e^{2\pi i\tau })))}$

where ${\displaystyle \operatorname {Im} \tau >0}$ and ${\displaystyle (e^{z})^{1/5))$ in the continued fraction should be evaluated as ${\displaystyle e^{z/5))$. The function ${\displaystyle \tau \mapsto R(e^{2\pi i\tau })}$ is invariant under ${\displaystyle \Gamma (5)}$, a congruence subgroup of the modular group. Also for positive real numbers ${\displaystyle a,b\in \mathbb {R} ^{+))$ and ${\displaystyle ab=\pi ^{2},}$ then[56]

${\displaystyle (\varphi +R(e^{-2a}))(\varphi +R(e^{-2b}))=\varphi {\sqrt {5))}$

and

${\displaystyle (\varphi ^{-1}-R(-e^{-a}))(\varphi ^{-1}-R(-e^{-b}))=\varphi ^{-1}{\sqrt {5)).}$

The golden ratio also appears in hyperbolic geometry, as the maximum distance from a point on one side of an ideal triangle to the closer of the other two sides: this distance, the side length of the equilateral triangle formed by the points of tangency of a circle inscribed within the ideal triangle, is ${\displaystyle 4\mathbin {:} \log(\varphi ).}$[57]

## Applications and observations

### Architecture

The Swiss architect Le Corbusier, famous for his contributions to the modern international style, centered his design philosophy on systems of harmony and proportion. Le Corbusier's faith in the mathematical order of the universe was closely bound to the golden ratio and the Fibonacci series, which he described as "rhythms apparent to the eye and clear in their relations with one another. And these rhythms are at the very root of human activities. They resound in man by an organic inevitability, the same fine inevitability which causes the tracing out of the Golden Section by children, old men, savages and the learned."[58][59]

Le Corbusier explicitly used the golden ratio in his Modulor system for the scale of architectural proportion. He saw this system as a continuation of the long tradition of Vitruvius, Leonardo da Vinci's "Vitruvian Man", the work of Leon Battista Alberti, and others who used the proportions of the human body to improve the appearance and function of architecture.

In addition to the golden ratio, Le Corbusier based the system on human measurements, Fibonacci numbers, and the double unit. He took suggestion of the golden ratio in human proportions to an extreme: he sectioned his model human body's height at the navel with the two sections in golden ratio, then subdivided those sections in golden ratio at the knees and throat; he used these golden ratio proportions in the Modulor system. Le Corbusier's 1927 Villa Stein in Garches exemplified the Modulor system's application. The villa's rectangular ground plan, elevation, and inner structure closely approximate golden rectangles.[60]

Another Swiss architect, Mario Botta, bases many of his designs on geometric figures. Several private houses he designed in Switzerland are composed of squares and circles, cubes and cylinders. In a house he designed in Origlio, the golden ratio is the proportion between the central section and the side sections of the house.[61]

### Art

Divina proportione (Divine proportion), a three-volume work by Luca Pacioli, was published in 1509. Pacioli, a Franciscan friar, was known mostly as a mathematician, but he was also trained and keenly interested in art. Divina proportione explored the mathematics of the golden ratio. Though it is often said that Pacioli advocated the golden ratio's application to yield pleasing, harmonious proportions, Livio points out that the interpretation has been traced to an error in 1799, and that Pacioli actually advocated the Vitruvian system of rational proportions.[62] Pacioli also saw Catholic religious significance in the ratio, which led to his work's title.

Leonardo da Vinci's illustrations of polyhedra in Divina proportione[63] have led some to speculate that he incorporated the golden ratio in his paintings. But the suggestion that his Mona Lisa, for example, employs golden ratio proportions, is not supported by Leonardo's own writings.[64] Similarly, although the Vitruvian Man is often shown in connection with the golden ratio, the proportions of the figure do not actually match it, and the text only mentions whole number ratios.[65][66]

Salvador Dalí, influenced by the works of Matila Ghyka,[67] explicitly used the golden ratio in his masterpiece, The Sacrament of the Last Supper. The dimensions of the canvas are a golden rectangle. A huge dodecahedron, in perspective so that edges appear in golden ratio to one another, is suspended above and behind Jesus and dominates the composition.[64][68]

A statistical study on 565 works of art of different great painters, performed in 1999, found that these artists had not used the golden ratio in the size of their canvases. The study concluded that the average ratio of the two sides of the paintings studied is ${\displaystyle 1.34,}$ with averages for individual artists ranging from ${\displaystyle 1.04}$ (Goya) to ${\displaystyle 1.46}$ (Bellini).[69] On the other hand, Pablo Tosto listed over 350 works by well-known artists, including more than 100 which have canvasses with golden rectangle and ${\displaystyle {\sqrt {5))}$ proportions, and others with proportions like ${\displaystyle {\sqrt {2)),}$ ${\displaystyle 3,}$ ${\displaystyle 4,}$ and ${\displaystyle 6.}$[70]

### Books and design

According to Jan Tschichold,

There was a time when deviations from the truly beautiful page proportions ${\displaystyle 2\mathbin {:} 3,}$ ${\displaystyle 1\mathbin {:} {\sqrt {3)),}$ and the Golden Section were rare. Many books produced between 1550 and 1770 show these proportions exactly, to within half a millimeter.[72]

According to some sources, the golden ratio is used in everyday design, for example in the proportions of playing cards, postcards, posters, light switch plates, and widescreen televisions.[73][74][75][76]

### Flags

The aspect ratio (height to width ratio) of the flag of Togo is in the golden ratio.

### Music

Ernő Lendvai analyzes Béla Bartók's works as being based on two opposing systems, that of the golden ratio and the acoustic scale,[77] though other music scholars reject that analysis.[78] French composer Erik Satie used the golden ratio in several of his pieces, including Sonneries de la Rose+Croix. The golden ratio is also apparent in the organization of the sections in the music of Debussy's Reflets dans l'eau (Reflections in Water), from Images (1st series, 1905), in which "the sequence of keys is marked out by the intervals 34, 21, 13 and 8, and the main climax sits at the phi position".[79]

The musicologist Roy Howat has observed that the formal boundaries of Debussy's La Mer correspond exactly to the golden section.[80] Trezise finds the intrinsic evidence "remarkable", but cautions that no written or reported evidence suggests that Debussy consciously sought such proportions.[81]

Though Heinz Bohlen proposed the non-octave-repeating 833 cents scale based on combination tones, the tuning features relations based on the golden ratio. As a musical interval the ratio 1.618... is 833.090... cents ().[82]

### Nature

Johannes Kepler wrote that "the image of man and woman stems from the divine proportion. In my opinion, the propagation of plants and the progenitive acts of animals are in the same ratio".[83]

The psychologist Adolf Zeising noted that the golden ratio appeared in phyllotaxis and argued from these patterns in nature that the golden ratio was a universal law.[84][85] Zeising wrote in 1854 of a universal orthogenetic law of "striving for beauty and completeness in the realms of both nature and art".[86]

However, some have argued that many apparent manifestations of the golden ratio in nature, especially in regard to animal dimensions, are fictitious.[87]

### Physics

The quasi-one-dimensional Ising ferromagnet CoNb2O6 (cobalt niobate) has 8 predicted excitation states (with E8 symmetry), that when probed with neutron scattering, showed its lowest two were in golden ratio. Specifically, these quantum phase transitions during spin excitation, which occur at near absolute zero temperature, showed pairs of kinks in its ordered-phase to spin-flips in its paramagnetic phase; revealing, just below its critical field, a spin dynamics with sharp modes at low energies approaching the golden mean.[88]

### Optimization

There is no known general algorithm to arrange a given number of nodes evenly on a sphere, for any of several definitions of even distribution (see, for example, Thomson problem or Tammes problem). However, a useful approximation results from dividing the sphere into parallel bands of equal surface area and placing one node in each band at longitudes spaced by a golden section of the circle, i.e. ${\displaystyle 360^{\circ }/\varphi \approx 222.5^{\circ }.}$ This method was used to arrange the 1500 mirrors of the student-participatory satellite Starshine-3.[89]

The golden ratio is a critical element to golden-section search as well.

## Disputed observations

Examples of disputed observations of the golden ratio include the following:

• Some specific proportions in the bodies of many animals (including humans)[90][91] and parts of the shells of mollusks[4] are often claimed to be in the golden ratio. There is a large variation in the real measures of these elements in specific individuals, however, and the proportion in question is often significantly different from the golden ratio.[90] The ratio of successive phalangeal bones of the digits and the metacarpal bone has been said to approximate the golden ratio.[91] The nautilus shell, the construction of which proceeds in a logarithmic spiral, is often cited, usually with the idea that any logarithmic spiral is related to the golden ratio, but sometimes with the claim that each new chamber is golden-proportioned relative to the previous one.[92] However, measurements of nautilus shells do not support this claim.[93]
• Historian John Man states that both the pages and text area of the Gutenberg Bible were "based on the golden section shape". However, according to his own measurements, the ratio of height to width of the pages is ${\displaystyle 1.45.}$[94]
• Studies by psychologists, starting with Gustav Fechner c. 1876,[95] have been devised to test the idea that the golden ratio plays a role in human perception of beauty. While Fechner found a preference for rectangle ratios centered on the golden ratio, later attempts to carefully test such a hypothesis have been, at best, inconclusive.[96][64]
• In investing, some practitioners of technical analysis use the golden ratio to indicate support of a price level, or resistance to price increases, of a stock or commodity; after significant price changes up or down, new support and resistance levels are supposedly found at or near prices related to the starting price via the golden ratio.[97] The use of the golden ratio in investing is also related to more complicated patterns described by Fibonacci numbers (e.g. Elliott wave principle and Fibonacci retracement). However, other market analysts have published analyses suggesting that these percentages and patterns are not supported by the data.[98]

### Egyptian pyramids

The Great Pyramid of Giza (also known as the Pyramid of Cheops or Khufu) has been analyzed by pyramidologists as having a doubled Kepler triangle as its cross-section. If this theory were true, the golden ratio would describe the ratio of distances from the midpoint of one of the sides of the pyramid to its apex, and from the same midpoint to the center of the pyramid's base. However, imprecision in measurement caused in part by the removal of the outer surface of the pyramid makes it impossible to distinguish this theory from other numerical theories of the proportions of the pyramid, based on pi or on whole-number ratios. The consensus of modern scholars is that this pyramid's proportions are not based on the golden ratio, because such a basis would be inconsistent both with what is known about Egyptian mathematics from the time of construction of the pyramid, and with Egyptian theories of architecture and proportion used in their other works.[99][100][101][102]

### The Parthenon

The Parthenon's façade (c. 432 BC) as well as elements of its façade and elsewhere are said by some to be circumscribed by golden rectangles.[104] Other scholars deny that the Greeks had any aesthetic association with golden ratio. For example, Keith Devlin says, "Certainly, the oft repeated assertion that the Parthenon in Athens is based on the golden ratio is not supported by actual measurements. In fact, the entire story about the Greeks and golden ratio seems to be without foundation."[105] Midhat J. Gazalé affirms that "It was not until Euclid ... that the golden ratio's mathematical properties were studied."[106]

From measurements of 15 temples, 18 monumental tombs, 8 sarcophagi, and 58 grave stelae from the fifth century BC to the second century AD, one researcher concluded that the golden ratio was totally absent from Greek architecture of the classical fifth century BC, and almost absent during the following six centuries.[107] Later sources like Vitruvius (first century BC) exclusively discuss proportions that can be expressed in whole numbers, i.e. commensurate as opposed to irrational proportions.

### Modern art

The Section d'Or ('Golden Section') was a collective of painters, sculptors, poets and critics associated with Cubism and Orphism.[108] Active from 1911 to around 1914, they adopted the name both to highlight that Cubism represented the continuation of a grand tradition, rather than being an isolated movement, and in homage to the mathematical harmony associated with Georges Seurat.[109] The Cubists observed in its harmonies, geometric structuring of motion and form, the primacy of idea over nature, an absolute scientific clarity of conception.[110] However, despite this general interest in mathematical harmony, whether the paintings featured in the celebrated 1912 Salon de la Section d'Or exhibition used the golden ratio in any compositions is more difficult to determine. Livio, for example, claims that they did not,[111] and Marcel Duchamp said as much in an interview.[112] On the other hand, an analysis suggests that Juan Gris made use of the golden ratio in composing works that were likely, but not definitively, shown at the exhibition.[112][113][114] Art historian Daniel Robbins has argued that in addition to referencing the mathematical term, the exhibition's name also refers to the earlier Bandeaux d'Or group, with which Albert Gleizes and other former members of the Abbaye de Créteil had been involved.[115]

Piet Mondrian has been said to have used the golden section extensively in his geometrical paintings,[116] though other experts (including critic Yve-Alain Bois) have discredited these claims.[64][117]

## References

### Explanatory footnotes

1. ^ If the constraint on ${\displaystyle a}$ and ${\displaystyle b}$ each being greater than zero is lifted, then there are actually two solutions, one positive and one negative, to this equation. ${\displaystyle \varphi }$ is defined as the positive solution. The negative solution is ${\displaystyle -\varphi ^{-1}={\tfrac {1}{2)){\bigl (}1-{\sqrt {5)){\bigr )}.}$ The sum of the two solutions is ${\displaystyle 1,}$ and the product of the two solutions is ${\displaystyle -1.}$
2. ^ Euclid, Elements, Book II, Proposition 11; Book IV, Propositions 10–11; Book VI, Proposition 30; Book XIII, Propositions 1–6, 8–11, 16–18.
3. ^ "῎Ακρον καὶ μέσον λόγον εὐθεῖα τετμῆσθαι λέγεται, ὅταν ᾖ ὡς ἡ ὅλη πρὸς τὸ μεῖζον τμῆμα, οὕτως τὸ μεῖζον πρὸς τὸ ἔλαττὸν."[20]
4. ^ After Classical Greek sculptor Phidias (c. 490–430 BC);[32] Barr later wrote that he thought it unlikely that Phidias actually used the golden ratio.[33]
5. ^ Not to be confused with the silver mean, also known as the silver ratio.
6. ^ Not to be confused with the congruence subgroup ${\displaystyle \Gamma (5).}$

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