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In mathematics, the supersilver ratio is a geometrical proportion close to 75/34. Its true value is the real solution of the equation x3 = 2x2 + 1.

Supersilver ratio
A supersilver rectangle contains two scaled copies of itself, ς = ((ς − 1)2 + 2(ς − 1) + 1) / ς
Rationalityirrational algebraic
Symbolς
Representations
Decimal2.2055694304005903117020286...
Algebraic formreal root of x3 = 2x2 + 1
Continued fraction (linear)[2;4,1,6,2,1,1,1,1,1,1,2,2,1,2,1,...]
not periodic
infinite

The name supersilver ratio results from analogy with the silver ratio, the positive solution of the equation x2 = 2x + 1, and the supergolden ratio.

Definition

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Two quantities a > b > 0 are in the supersilver ratio-squared if

 .

The ratio   is here denoted  

Based on this definition, one has

 

It follows that the supersilver ratio is found as the unique real solution of the cubic equation   The decimal expansion of the root begins as   (sequence A356035 in the OEIS).

The minimal polynomial for the reciprocal root is the depressed cubic   thus the simplest solution with Cardano's formula,

 
 

or, using the hyperbolic sine,

 

  is the superstable fixed point of the iteration  

Rewrite the minimal polynomial as  , then the iteration   results in the continued radical

 [1]

Dividing the defining trinomial   by   one obtains  , and the conjugate elements of   are

 

with   and  

Properties

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Rectangles with aspect ratios related to powers of ς tile the square.

The growth rate of the average value of the n-th term of a random Fibonacci sequence is  .[2]

The supersilver ratio can be expressed in terms of itself as the infinite geometric series

  and  

in comparison to the silver ratio identities

  and  

For every integer   one has

 

Continued fraction pattern of a few low powers

  (5/24)
  (5/11)
 
  (53/24)
  (73/15)
  (118/11)

The supersilver ratio is a Pisot number.[3] Because the absolute value   of the algebraic conjugates is smaller than 1, powers of   generate almost integers. For example:   After ten rotation steps the phases of the inward spiraling conjugate pair – initially close to   – 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   Thus, the Hilbert class field of   can be formed by adjoining  [4] With argument   a generator for the ring of integers of  , the real root  j(τ) of the Hilbert class polynomial is given by  [5][6]

The Weber-Ramanujan class invariant is approximated with error < 3.5 ∙ 10−20 by

 

while its true value is the single real root of the polynomial

 

The elliptic integral singular value[7]   for   has closed form expression

 

(which is less than 1/294 the eccentricity of the orbit of Venus).

Third-order Pell sequences

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Hop o' my Thumb: a supersilver Rauzy fractal of type a ↦ baa. The fractal boundary has box-counting dimension 1.22
 
A supersilver Rauzy fractal of type c ↦ bca, with areas in the ratios ς2 + 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

  for n > 2,

with initial values

 

The first few terms are 1, 2, 4, 9, 20, 44, 97, 214, 472, 1041, 2296, 5064,... (sequence A008998 in the OEIS). The limit ratio between consecutive terms is the supersilver ratio.

The first 8 indices n for which   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

  for  [8]

The third-order Pell numbers are related to sums of binomial coefficients by

 .[9]

The characteristic equation of the recurrence is   If the three solutions are real root   and conjugate pair   and  , the supersilver numbers can be computed with the Binet formula

  with real   and conjugates   and   the roots of  

Since   and   the number   is the nearest integer to   with n ≥ 0 and   0.1732702315504081807484794...

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,... (sequence A332647 in the OEIS).

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 108 to pass the test are n = 32, 52, 53, 315, 99297, 222443, 418625, 9122185, 32572, 11889745, 20909625, 24299681, 64036831, 76917325.[10]

 
The Pilgrim: a supersilver Rauzy fractal of type a ↦ aba. The three subtiles have areas in ratio ς.

The third-order Pell numbers are obtained as integral powers n > 3 of a matrix with real eigenvalue  

 
 

The trace of   gives the above  

Alternatively,   can be interpreted as incidence matrix for a D0L Lindenmayer system on the alphabet   with corresponding substitution rule

 

and initiator  . The series of words   produced by iterating the substitution have the property that the number of c's, b's and a's are equal to successive third-order Pell numbers. The lengths of these words are given by  [11]

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.[12]

Supersilver rectangle

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Powers of ς within a supersilver rectangle.

Given a rectangle of height 1, length   and diagonal length   The triangles on the diagonal have altitudes   each perpendicular foot divides the diagonal in ratio  .

On the right-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   (according to  ). Divide the original rectangle into four parts by a second, horizontal cut passing through the intersection point.[13]

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

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Supersilver spirals with different initial angles on a ς− rectangle.

A supersilver spiral is a logarithmic spiral that gets wider by a factor of   for every quarter turn. It is described by the polar equation   with initial radius   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 orthogonally aligned and successively scaled by a factor  


See also

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  • 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  

References

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  1. ^ Sloane, N. J. A. (ed.). "Sequence A272874". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  2. ^ (sequence A137421 in the OEIS)
  3. ^ Panju, Maysum (2011). "A systematic construction of almost integers" (PDF). The Waterloo Mathematics Review. 1 (2): 35–43.
  4. ^ "Hilbert class field of a quadratic field whose class number is 3". Mathematics stack exchange. 2012. Retrieved May 1, 2024.
  5. ^ Berndt, Bruce C.; Chan, Heng Huat (1999). "Ramanujan and the modular j-invariant". Canadian Mathematical Bulletin. 42 (4): 427–440. doi:10.4153/CMB-1999-050-1.
  6. ^ Johansson, Fredrik (2021). "Modular j-invariant". Fungrim. Retrieved April 30, 2024. Table of Hilbert class polynomials
  7. ^ Weisstein, Eric W. "Elliptic integral singular value". MathWorld.
  8. ^ (sequence A008998 in the OEIS)
  9. ^ Mahon, Br. J. M.; Horadam, A. F. (1990). "Third-order diagonal functions of Pell polynomials". The Fibonacci Quarterly. 28 (1): 3–10. doi:10.1080/00150517.1990.12429513.
  10. ^ Only one of these is a 'restricted pseudoprime' as defined in: Adams, William; Shanks, Daniel (1982). "Strong primality tests that are not sufficient". Mathematics of Computation. 39 (159). American Mathematical Society: 255–300. doi:10.1090/S0025-5718-1982-0658231-9. JSTOR 2007637.
  11. ^ for n ≥ 2 (sequence A193641 in the OEIS)
  12. ^ Siegel, Anne; Thuswaldner, Jörg M. (2009). "Topological properties of Rauzy fractals". Mémoires de la Société Mathématique de France. 2. 118: 1–140. doi:10.24033/msmf.430.
  13. ^ Analogue to the construction in: Crilly, Tony (1994). "A supergolden rectangle". The Mathematical Gazette. 78 (483): 320–325. doi:10.2307/3620208. JSTOR 3620208.