Supergolden ratio

Short description: Number, approximately 1.46557

Template:Infobox non-integer number In mathematics, the supergolden ratio is a geometrical proportion, given by the unique real solution of the equation x³ = x² + 1. Its decimal expansion begins with 1.465571231876768... (sequence A092526 in the OEIS).

The name supergolden ratio is by analogy with the golden ratio, the positive solution of the equation x² = x + 1.

Definition

ψ = ⁠a/b⁠ = ⁠a+c/a⁠ = ⁠b/c⁠. For b = 1 the boxes have volumes ψ³ = ψ² (red) + 1 (green).

Three quantities a > b > c > 0 are in the supergolden ratio if $$\frac{a+c}{a}=\frac{a}{b}=\frac{b}{c}$$ This common ratio is commonly denoted $\psi .$

Substituting $b=\psi c$ and $a=\psi b={\psi }^{2}c$ in the first fraction, $$\psi =\frac{c({\psi }^2+1)}{{\psi }^{2}c}.$$ It follows that the supergolden ratio is the unique real solution of the cubic equation ${\psi }^3-{\psi }^2-1=0$.

The minimal polynomial for the reciprocal root is the depressed cubic $x^3+x-1$, thus the simplest solution with Cardano's formula, $$\begin{array}{rl}{w}_{1,2}& =(1\pm \frac{1}{3}\sqrt{\frac{31}{3}})/2\\ 1/\psi & =\sqrt[3]{w_1}+\sqrt[3]{w_2}\end{array}$$

or, using the hyperbolic sine, $$1/\psi =\frac{2}{\sqrt{3}}\sinh (\frac{1}{3}\mathrm{arsinh}\left(\frac{3\sqrt{3}}{2}\right)).$$

A triangle with side lengths ψ, 1, and 1 ∕ ψ has an angle of exactly 120 degrees.

$1/\psi$ is the superstable fixed point of the Newton's method iteration $x\leftarrow (2x^3+1)/(3x^2+1)$.

Dividing the defining trinomial $x^3-x^2-1$ by $x-\psi$ one obtains $x^2+x/{\psi }^2+1/\psi$, and the conjugate elements of $\psi$ are $$x_{1,2}=(-1\pm i\sqrt{4{\psi }^2+3})/2{\psi }^2,$$ with $x_1+x_2=1-\psi$ and $x_1{x}_2=1/\psi$.

The iteration $x\leftarrow \sqrt[3]{\phantom{|}1+x^2}$ results in the continued radical  $${\psi }^3-1={\psi }^2=\sqrt[3/2]{1+\sqrt[3/2]{1+\sqrt[3/2]{\phantom{|}1+\cdots }}}$$

Better convergence is attained by using relation $y^3=y(1-2y)+3$ with real zero ${\psi }^2-1$. Divide both sides by $y$, and substitute ${\psi }^{-3}=1/(y+2)\text{ for }3/y-2y.$ This gives the iteration $y\leftarrow \sqrt{1+\frac{1}{2+y}},$ and the continued reciprocal square root $${\psi }^2-1=\sqrt{1+\frac{1}{2+\sqrt{1+\frac{1}{2+\sqrt{1+\frac{1}{2+\ddots }}}}}}$$

Properties

Rectangles in aspect ratios ψ, ψ² and ψ³ (from left to right) tile the square.

The supergolden ratio can be expressed in terms of itself in many different ways, for example as fractions $$2\psi -1=\frac{{\psi }^2+2}{{\psi }^2}=\frac{{\psi }^3+1}{{\psi }^3-1}=\frac{2{\psi }^7-1}{{\psi }^7},$$ along with $$\begin{array}{rl}1+{\psi }^{-1}& =\frac{{\psi }^2+2}{\psi +1}=\frac{2{\psi }^3-1}{{\psi }^3}\\ 1+{\psi }^{-3}& =\frac{{\psi }^2+2}{{\psi }^2+1}\\ 1+{\psi }^{-5}& =\frac{\psi +1}{{\psi }^2}\\ ={\psi }^2-1& =\frac{{\psi }^7+1}{{\psi }^7-1}\\ {\psi }^4-1& =\frac{{\psi }^2+2}{{\psi }^2-1}\\ {\psi }^6-1& =\frac{{\psi }^2+2}{\psi -1}.\end{array}$$

Similarly as infinite geometric series $$\begin{array}{rl}{\psi }^3& =\sum _{n=0}^{\mathrm{\infty }}{\psi }^{-n}\\ \frac{{\psi }^2}{{\psi }^2-1}& =\sum _{n=0}^{\mathrm{\infty }}{\psi }^{-2n}\\ \psi & =\sum _{n=0}^{\mathrm{\infty }}{\psi }^{-3n}\\ \frac{\psi }{{\psi }^2-1}& =\sum _{n=0}^{\mathrm{\infty }}{\psi }^{-4n}\\ \frac{1}{3-{\psi }^2}& =\sum _{n=0}^{\mathrm{\infty }}{\psi }^{-5n}\\ \frac{1}{{\psi }^2-1}+\frac{1}{{\psi }^2+2}& =\sum _{n=0}^{\mathrm{\infty }}{\psi }^{-6n}\\ \frac{{\psi }^2}{2}& =\sum _{n=0}^{\mathrm{\infty }}{\psi }^{-7n}.\end{array}$$ Additionally, $\sum _{n=0}^7{\psi }^{-n}=3.$

For every integer $n$ one has $$\begin{array}{rl}{\psi }^n& ={\psi }^{n-1}+{\psi }^{n-3}\\ & ={\psi }^{n-2}+{\psi }^{n-3}+{\psi }^{n-4}\\ & ={\psi }^{n-2}+2{\psi }^{n-4}+{\psi }^{n-6}\end{array}$$ from this an infinite number of further relations can be found.

Argument $\theta =\mathrm{arcsec}(2{\psi }^4)$ satisfies the identity $\tan (\theta )-4\sin (\theta )=3\sqrt{3}$.

Continued fraction pattern of a few low powers $$\begin{array}{rl}{\psi }^{-1}& =[0;1,2,6,1,3,5,4,22,...]\approx 0.6823(\frac{13}{19})\\ {\psi }^0& =[1]\\ {\psi }^1& =[1;2,6,1,3,5,4,22,1,...]\approx 1.4656(\frac{22}{15})\\ {\psi }^2& =[2;6,1,3,5,4,22,1,1,...]\approx 2.1479(\frac{15}{7})\\ {\psi }^3& =[3;6,1,3,5,4,22,1,1,...]\approx 3.1479(\frac{22}{7})\\ {\psi }^4& =[4;1,1,1,1,2,2,1,2,2,...]\approx 4.6135(\frac{60}{13})\\ {\psi }^5& =[6;1,3,5,4,22,1,1,4,...]\approx 6.7614(\frac{115}{17})\end{array}$$

Notably, the continued fraction of ${\psi }^2$ begins as permutation of the first six natural numbers; the next term is equal to their sum + 1.

As derived from its continued fraction expansion, the simplest rational approximations of $\psi$ are: $$\frac{3}{2},\frac{19}{13},\frac{22}{15},\frac{85}{58},\frac{277}{189},\frac{447}{305},\frac{1873}{1278},\frac{41653}{28421},\frac{43526}{29699},\frac{85179}{58120},\dots$$

Newton's method for p(z) = z³ − z² − 1: ψ (right) and its complex conjugates at the nuclei of their basins of attraction. Julia set of the Newton map in orange, with unit circle and real curve for reference.

The supergolden ratio is the fourth smallest Pisot number. By definition of these numbers, the absolute value $1/\sqrt{\psi }$ of the algebraic conjugates is smaller than 1, thus powers of $\psi$ generate almost integers. For example: ${\psi }^{11}=67.000222765...$ $\approx 67+1/4489$. After eleven rotation steps the phases of the inward spiraling conjugate pair – initially close to $\pm 13\pi /22$ – nearly align with the imaginary axis.

The minimal polynomial of the supergolden ratio $m(x)=x^3-x^2-1$ has discriminant $\mathrm{\Delta }=-31.$ The Hilbert class field of imaginary quadratic field $K=\mathbb{\mathbb{Q}}(\sqrt{\mathrm{\Delta }})$ can be formed by adjoining $\psi .$ With argument $\tau =(1+\sqrt{\mathrm{\Delta }})/2$ a generator for the ring of integers of $K$, one has the special value of Dedekind eta quotient $$\psi =\frac{e^{\pi i/24}\eta (\tau )}{\sqrt{2}\eta (2\tau )}.$$

Expressed in terms of the Weber-Ramanujan class invariant G_n  $$\psi =\frac{\mathfrak{𝔣}(\sqrt{\mathrm{\Delta }})}{\sqrt{2}}=\frac{G_{31}}{\sqrt[4]{2}}.$$

Properties of the related Klein j-invariant $j(\tau )$ result in near identity $e^{\pi \sqrt{-\mathrm{\Delta }}}\approx {\left(\sqrt{2}\psi \right)}^{24}-24.$ The difference is < 1/143092.

The elliptic integral singular value  $k_r={\lambda }^{*}(r)$ for $r=31$ has closed form expression $${\lambda }^{*}(31)=\sin (\arcsin ((\sqrt[4]{2}\psi {)}^{-12})/2)$$ (which is less than 1/10 the eccentricity of the orbit of Venus).

Narayana sequence

A Rauzy fractal associated with the supergolden ratio-cubed. The central tile and its three subtiles have areas in the ratios ψ⁴ : ψ² : ψ : 1.

A Rauzy fractal associated with the supergolden ratio-squared, with areas as above.

Narayana's cows is a recurrence sequence originating from a problem proposed by the 14th century Indian mathematician Narayana Pandita. He asked for the number of cows and calves in a herd after 20 years, beginning with one cow in the first year, where each cow gives birth to one calf each year from the age of three onwards.

The Narayana sequence has a close connection to the Fibonacci and Padovan sequences and plays an important role in data coding, cryptography and combinatorics. The number of compositions of n into parts 1 and 3 is counted by the nth Narayana number.

The Narayana sequence is defined by the third-order recurrence relation $$N_n=N_{n-1}+N_{n-3}\text{ for }n>2,$$ with initial values $$N_0=N_1=N_2=1.$$

The first few terms are 1, 1, 1, 2, 3, 4, 6, 9, 13, 19, 28, 41, 60, 88,... (sequence A000930 in the OEIS). The limit ratio between consecutive terms is the supergolden ratio:$\underset{n\to \mathrm{\infty }}{\lim }{N}_{n+1}/N_n=\psi .$

The first 11 indices n for which $N_n$ is prime are n = 3, 4, 8, 9, 11, 16, 21, 25, 81, 6241, 25747 (sequence A170954 in the OEIS). The last number has 4274 decimal digits.

The sequence is extended to negative indices using $$N_n=N_{n+3}-N_{n+2},$$ obtaining (1), 0, 0, 1, 0,−1, 1, 1,−2, 0, 3,−2,−3, 5,...

Powers of the supergolden ratio can be written with Narayana numbers as quadratic coefficients $${\psi }^n={\psi }^2{N}_{n-2}+\psi N_{n-4}+N_{n-3},$$ which is proved by mathematical induction on $n.$ This relation also holds for $n<0.$ The order of the coefficients corresponds to the bottom row of matrix $Q$ below.

The generating function of the Narayana sequence for non-negative $n$ is given by $$\frac{1}{1-x-x^3}=\sum _{n=0}^{\mathrm{\infty }}{N}_n{x}^n\text{ for }x<\frac{1}{\psi }$$

The Narayana numbers are related to sums of binomial coefficients by $$N_n=\sum _{k=0}^{\lfloor n/3\rfloor }(\genfrac{}{}{0ex}{}{n-2k}{k})$$

The characteristic equation of the recurrence is $x^3-x^2-1=0.$ If the three solutions are real root $\psi$ and conjugate pair $\beta$ and $\gamma$, the Narayana numbers are given by the Binet formula  $$N_{n-1}=a{\psi }^n+b{\beta }^n+c{\gamma }^n,$$ with real $a$ and conjugates $b$ and $c$ the roots of $31x^3-3x-1=0.$

Since $|b{\beta }^n+c{\gamma }^n|<{\psi }^{-n/2}$, the number $N_n$ is the nearest integer to $a{\psi }^{n+1}$, with $n\ge 0$ and coefficient $a={\psi }^2/({\psi }^3+2)=$ 0.4172379879262187776214755...

Coefficients $a=b=c=1$ result in the Binet formula for the related sequence $A_n=N_n+2N_{n-3}.$

The first few terms are 3, 1, 1, 4, 5, 6, 10, 15, 21, 31, 46, 67, 98, 144,... (sequence A001609 in the OEIS).

This anonymous sequence has the Fermat property: if p is prime, $A_p\equiv A_{1}modp.$ The converse does not hold, but the small number of odd pseudoprimes $n\mid (A_n-1)$ makes the sequence special. The 8 odd composite numbers below 10⁸ to pass the test are n = 1155, 552599, 2722611, 4822081, 10479787, 10620331, 16910355, 66342673.

A supergolden Rauzy fractal of type a ↦ ab, with areas as above. The fractal boundary has box-counting dimension 1.50

The Narayana numbers are obtained as integral powers n > 3 of a matrix with real eigenvalue $\psi$ $$Q=\left(\begin{array}{ccc}1& 0& 1\\ 1& 0& 0\\ 0& 1& 0\end{array}\right),$$

$$Q^n=\left(\begin{array}{ccc}{N}_n& N_{n-2}& N_{n-1}\\ N_{n-1}& N_{n-3}& N_{n-2}\\ N_{n-2}& N_{n-4}& N_{n-3}\end{array}\right)$$

The trace of $Q^n$ gives the above $A_n$.

Alternatively, $Q$ can be interpreted as incidence matrix for a D0L Lindenmayer system on the alphabet $\{a,b,c\}$ with corresponding substitution rule $$\{\begin{array}{l}a\mapsto ab\\ b\mapsto c\\ c\mapsto a\end{array}$$ and initiator $w_0=b$. The series of words $w_n$ produced by iterating the substitution have lengths $l(w_n)=N_n$. The number of c, b and a's in each word is equal to successive Narayana numbers. Associated to this string rewriting process is a compact set composed of self-similar tiles called the Rauzy fractal, that visualizes the combinatorial information contained in a multiple-generation three-letter sequence.

The ratio between the number of digits in successive strings for the binary version of the look-and-say sequence converges to $\psi$.

Geometry

Psi-quad substitution tiling

Psi-quad substitution tiling.

Supergolden fractal triangle.

In the complex plane, if $\psi$ is a non-real root of ${\psi }^3-{\psi }^2-1=0$, then the vertices of a self-similar quadrilateral tiling (the psi-quad substitution tiling) can be taken as the six points ${\psi }^0,{\psi }^1,{\psi }^2,{\psi }^3,{\psi }^4,-{\psi }^2$. The iterated boundary is the supergolden fractal triangle, which has Hausdorff dimension $d_{\partial }=\frac{2\log \zeta }{\log \psi }$, where $\zeta$ is the real root of ${\zeta }^7-2{\zeta }^2-1=0$, so numerically $d_{\partial }\approx 1.0295240599$.

Supergolden rectangle

Nested supergolden rectangles with perpendicular diagonals and side lengths in powers of ψ.

A supergolden rectangle is a rectangle whose side lengths are in a $\psi :1$ ratio. Compared to the golden rectangle, the supergolden rectangle has one more degree of self-similarity.

Given a rectangle of height 1, length $\psi$ and diagonal length $\sqrt{{\psi }^3}$ (according to $1+{\psi }^2={\psi }^3$). The triangles on the diagonal have altitudes $1/\sqrt{\psi };$ each perpendicular foot divides the diagonal in ratio ${\psi }^2$.

On the left-hand side, cut off a square of side length 1 and mark the intersection with the falling diagonal. The remaining rectangle now has aspect ratio ${\psi }^2:1$ (according to $\psi -1={\psi }^{-2}$). Divide the original rectangle into four parts by a second, horizontal cut passing through the intersection point.

The rectangle below the diagonal has aspect ratio ${\psi }^3$, the other three are all supergolden rectangles, with a fourth one between the feet of the altitudes. The parent rectangle and the four scaled copies have linear sizes in the ratios ${\psi }^3:{\psi }^2:\psi :{\psi }^2-1:1.$ It follows from the theorem of the gnomon that the areas of the two rectangles opposite the diagonal are equal.

In the supergolden rectangle above the diagonal, the process is repeated at a scale of $1:{\psi }^2$.

Supergolden spiral

Supergolden spirals with different initial radii on a ψ− rectangle.

A supergolden spiral is a logarithmic spiral that gets wider by a factor of $\psi$ for every quarter turn. It is described by the polar equation $r(\theta )=a\exp (k\theta ),$ with initial radius $a$ and parameter $k=\frac{2}{\pi }\ln (\psi ).$ If drawn on a supergolden rectangle, the spiral has its pole at the foot of altitude of a triangle on the diagonal and passes through vertices of rectangles with aspect ratio $\psi$ which are perpendicularly aligned and successively scaled by a factor ${\psi }^{-1}.$

See also

Solutions of equations similar to $x^3=x^2+1$:

• Golden ratio – the positive solution of the equation $x^2=x+1$
• Plastic ratio – the real solution of the equation $x^3=x+1$
• Supersilver ratio – the real solution of the equation $x^3=2x^2+1$