Supersilver ratio#Third-order Pell sequences
{{short description|Number, approximately 2.20557}}
{{infobox non-integer number
| image=Supersilver rectangle RBGY.png
| image_caption=A supersilver rectangle contains two scaled copies of itself, {{math|1=ς = ((ς − 1){{sup|2}} + 2(ς − 1) + 1) / ς }}
| rationality=irrational algebraic
| symbol={{math|ς}}
| decimal={{val|2.20556943040059031170|end=...}}
| continued_fraction_linear={{math|[2;4,1,6,2,1,1,1,1,1,1,2,2,1,2,1,...]}} {{cite OEIS|A376121}}
| continued_fraction_periodic=not periodic
| continued_fraction_finite=infinite
| algebraic=real root of {{math|1=x{{isup|3}} = 2x{{isup|2}} + 1}}
}}
In mathematics, the supersilver ratio is a geometrical proportion close to {{math|75/34}}. It is the unique real solution of the equation {{nowrap|{{math|1=x{{isup|3}} = 2x{{isup|2}} + 1}}.}} The decimal expansion of the root begins as {{math|{{val|2.205569430400590|end=...}} }} {{OEIS|A356035}}.
The name supersilver ratio results from analogy with the silver ratio, the positive solution of the equation {{math|1=x{{isup|2}} = 2x + 1}}, and the supergolden ratio.
Definition
Three quantities {{math|a > b > c > 0}} are in the supersilver ratio if
The ratio {{tmath|\frac{a}{b} }} is commonly denoted {{tmath|\varsigma.}}
Substituting and in the third fraction,
It follows that the supersilver ratio is the unique real solution of the cubic equation
The minimal polynomial for the reciprocal root is the depressed cubic thus the simplest solution with Cardano's formula,
w_{1,2} &=\left( 1 \pm \frac{1}{3} \sqrt{ \frac{59}{3}} \right) /2 \\
1 /\varsigma &=\sqrt[3]{w_1} +\sqrt[3]{w_2} \end{align}
or, using the hyperbolic sine,
:
{{tmath|1 /\varsigma}} is the superstable fixed point of the iteration
Rewrite the minimal polynomial as , then the iteration results in the continued radical
Dividing the defining trinomial by {{tmath|x -\varsigma}} one obtains , and the conjugate elements of {{tmath|\varsigma}} are
with and
Properties
The growth rate of the average value of the n-th term of a random Fibonacci sequence is {{tmath|\varsigma - 1}}.{{OEIS|A137421}}
The defining equation can be written
1 &=\frac{1}{\varsigma -1} +\frac{1}{\varsigma^2 +1} \\
&=\frac{1}{\varsigma} +\frac{\varsigma -1}{\varsigma +1} +\frac{\varsigma -2}{\varsigma -1}.\end{align}
The supersilver ratio can be expressed in terms of itself as fractions
\varsigma &=\frac{\varsigma}{\varsigma -1} +\frac{\varsigma -1}{\varsigma +1} \\
\varsigma^2 &=\frac{1}{\varsigma -2}.\end{align}
Similarly as the infinite geometric series
\varsigma &=2\sum_{n=0}^{\infty} \varsigma^{-3n} \\
\varsigma^2 &=-1 +\sum_{n=0}^{\infty} (\varsigma -1)^{-n},\end{align}
in comparison to the silver ratio identities
\sigma &=2\sum_{n=0}^{\infty} \sigma^{-2n} \\
\sigma^2 &=-1 +2\sum_{n=0}^{\infty} (\sigma -1)^{-n}.\end{align}
For every integer {{tmath|n}} one has
\varsigma^{n} &=2\varsigma^{n-1} +\varsigma^{n-3} \\
&=4\varsigma^{n-2} +\varsigma^{n-3} +2\varsigma^{n-4} \\
&=\varsigma^{n-1} +2\varsigma^{n-2} +\varsigma^{n-3} +\varsigma^{n-4} \end{align}
From this an infinite number of further relations can be found.
Continued fraction pattern of a few low powers
\varsigma^{-2} &=[0;4,1,6,2,1,1,1,1,1,1,...] \approx 0.2056 \;(\tfrac{5}{24}) \\
\varsigma^{-1} &=[0;2,4,1,6,2,1,1,1,1,1,...] \approx 0.4534 \;(\tfrac{5}{11}) \\
\varsigma^0 &=[1] \\
\varsigma^1 &=[2;4,1,6,2,1,1,1,1,1,1,...] \approx 2.2056 \;(\tfrac{53}{24}) \\
\varsigma^2 &=[4;1,6,2,1,1,1,1,1,1,2,...] \approx 4.8645 \;(\tfrac{73}{15}) \\
\varsigma^3 &=[10;1,2,1,2,4,4,2,2,6,2,...] \approx 10.729 \;(\tfrac{118}{11}) \end{align}
The simplest rational approximations of {{tmath|\varsigma}} are:
The supersilver ratio is a Pisot number.{{cite journal |last=Panju |first=Maysum |date=2011 |title=A systematic construction of almost integers |url=https://mathreview.uwaterloo.ca/archive/voli/2/panju.pdf |journal=The Waterloo Mathematics Review |volume=1 |issue=2 |pages=35–43}} Because the absolute value of the algebraic conjugates is smaller than 1, powers of {{tmath|\varsigma}} generate almost integers. For example: After ten rotation steps the phases of the inward spiraling conjugate pair – initially close to {{tmath|\pm 45 \pi/82}} – nearly align with the imaginary axis.
The minimal polynomial of the supersilver ratio has discriminant and factors into the imaginary quadratic field has class number {{tmath|h{{=}}3.}} Thus, the Hilbert class field of {{tmath|K}} can be formed by adjoining {{tmath|\varsigma.}}{{cite web| url=https://math.stackexchange.com/questions/184423/ |title=Hilbert class field of a quadratic field whose class number is 3 |date=2012 |website=Mathematics stack exchange |access-date=May 1, 2024}}
With argument a generator for the ring of integers of {{tmath|K}}, the real root J-invariant of the Hilbert class polynomial is given by {{cite journal |last1=Berndt |first1=Bruce C.|last2=Chan |first2=Heng Huat |date=1999 |title=Ramanujan and the modular j-invariant |journal=Canadian Mathematical Bulletin |volume=42 |issue=4 |pages=427–440 |doi=10.4153/CMB-1999-050-1 |doi-access=free}}{{cite web |url=https://fungrim.org/topic/Modular_j-invariant/ |title=Modular j-invariant |last=Johansson |first=Fredrik |date=2021 |website=Fungrim |access-date=April 30, 2024 |quote=Table of Hilbert class polynomials}}
The Weber-Ramanujan class invariant is approximated with error {{math|< 3.5 ∙ 10−20}} by
:
while its true value is the single real root of the polynomial
:
The elliptic integral singular value{{MathWorld |id=EllipticIntegralSingularValue |title=Elliptic integral singular value}} has closed form expression
:
(which is less than 1/294 the eccentricity of the orbit of Venus).
Third-order Pell sequences
{{multiple image
| total_width=320
| direction=vertical
| image_gap=6
| image1=Supersilver_Rauzy_baa.png
| caption1=Hop o' my Thumb: a supersilver Rauzy fractal of type {{nowrap|a ↦ baa.}} The fractal boundary has box-counting dimension 1.22
| image2=Supersilver_Rauzy_bca.png
| caption2=A supersilver Rauzy fractal of type {{nowrap|c ↦ bca,}} with areas in the ratios {{math|ς{{sup|2}} + 1 : ς (ς − 1) : ς : 1.}}
}}
These numbers are related to the supersilver ratio as the Pell numbers and Pell-Lucas numbers are to the silver ratio.
The fundamental sequence is defined by the third-order recurrence relation
with initial values
The first few terms are 1, 2, 4, 9, 20, 44, 97, 214, 472, 1041, 2296, 5064,... {{OEIS|A008998}}.
The limit ratio between consecutive terms is the supersilver ratio:
The first 8 indices n for which {{tmath|S_{n} }} is prime are n = 1, 6, 21, 114, 117, 849, 2418, 6144. The last number has 2111 decimal digits.
The sequence can be extended to negative indices using
The generating function of the sequence is given by
The third-order Pell numbers are related to sums of binomial coefficients by
:.{{Cite journal |last1=Mahon |first1=Br. J. M. |last2=Horadam |first2=A. F. |date=1990 |title=Third-order diagonal functions of Pell polynomials |journal=The Fibonacci Quarterly |volume=28 |issue=1 |pages=3–10|doi=10.1080/00150517.1990.12429513 }}
The characteristic equation of the recurrence is If the three solutions are real root {{tmath|\alpha}} and conjugate pair {{tmath|\beta}} and {{tmath|\gamma}}, the supersilver numbers can be computed with the Binet formula
with real {{tmath|a}} and conjugates {{tmath|b}} and {{tmath|c}} the roots of
Since and the number {{tmath|S_{n} }} is the nearest integer to with {{math|n ≥ 0}} and {{gaps|0.17327|02315|50408|18074|84794...}}
Coefficients result in the Binet formula for the related sequence
The first few terms are 3, 2, 4, 11, 24, 52, 115, 254, 560, 1235, 2724, 6008,... {{OEIS|A332647}}.
This third-order Pell-Lucas sequence has the Fermat property: if p is prime, The converse does not hold, but the small number of odd pseudoprimes makes the sequence special. The 14 odd composite numbers below {{math|10{{sup|8}} }} to pass the test are n = 3{{sup|2}}, 5{{sup|2}}, 5{{sup|3}}, 315, 99297, 222443, 418625, 9122185, 3257{{sup|2}}, 11889745, 20909625, 24299681, 64036831, 76917325.Only one of these is a 'restricted pseudoprime' as defined in: {{cite journal |last1=Adams |first1=William |last2=Shanks |first2=Daniel |author-link2=Daniel Shanks |title=Strong primality tests that are not sufficient |journal=Mathematics of Computation |publisher=American Mathematical Society |date=1982 |volume=39 |issue=159 |pages=255–300 |doi=10.1090/S0025-5718-1982-0658231-9 |doi-access=free| jstor=2007637}}
File:Supersilver_Rauzy_aba.png
The third-order Pell numbers are obtained as integral powers {{math|n > 3}} of a matrix with real eigenvalue {{tmath|\varsigma}}
The trace of {{tmath|Q^{n} }} gives the above {{tmath|A_{n}.}}
Alternatively, {{tmath|Q}} can be interpreted as incidence matrix for a D0L Lindenmayer system on the alphabet {{tmath|\{a,b,c\} }} with corresponding substitution rule
a \;\mapsto \;aab \\
b \;\mapsto \;c \\
c \;\mapsto \;a \end{cases}
and initiator {{tmath|w_0{{=}}b}}. The series of words {{tmath|w_n}} produced by iterating the substitution have the property that the number of {{math|c's, b's}} and {{math|a's}} are equal to successive third-order Pell numbers. The lengths of these words are given by for n ≥ 2 {{OEIS|A193641}}
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.{{cite journal |last1=Siegel |first1=Anne |last2=Thuswaldner |first2=Jörg M. |date=2009 |title=Topological properties of Rauzy fractals |url=http://numdam.org/item/MSMF_2009_2_118__1_0/ |journal=Mémoires de la Société Mathématique de France |volume=118 |series=2 |pages=1–140 |doi=10.24033/msmf.430}}
Supersilver rectangle
Given a rectangle of height {{math|1}}, length {{tmath|\varsigma}} and diagonal length The triangles on the diagonal have altitudes each perpendicular foot divides the diagonal in ratio {{tmath|\varsigma^2}}.
On the right-hand side, cut off a square of side length {{math|1}} and mark the intersection with the falling diagonal. The remaining rectangle now has aspect ratio (according to ). Divide the original rectangle into four parts by a second, horizontal cut passing through the intersection point.Analogue to the construction in: {{cite journal |last=Crilly |first=Tony |date=1994 |title=A supergolden rectangle |journal=The Mathematical Gazette |volume=78 |issue=483 |pages=320–325 |doi=10.2307/3620208|jstor=3620208 }}
The parent supersilver rectangle and the two scaled copies along the diagonal have linear sizes in the ratios The areas of the rectangles opposite the diagonal are both equal to with aspect ratios (below) and (above).
If the diagram is further subdivided by perpendicular lines through the feet of the altitudes, the lengths of the diagonal and its seven distinct subsections are in ratios
=Supersilver spiral=
A supersilver spiral is a logarithmic spiral that gets wider by a factor of {{tmath|\varsigma}} for every quarter turn. It is described by the polar equation with initial radius {{tmath|a}} and parameter If drawn on a supersilver 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 which are perpendicularly aligned and successively scaled by a factor
See also
- Solutions of equations similar to :
- Silver ratio – the only positive solution of the equation
- Golden ratio – the only positive solution of the equation
- Supergolden ratio – the only real solution of the equation