Metallic mean

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Gold, silver, and bronze ratios within their respective rectangles.

The metallic mean (also metallic ratio, metallic constant, or noble means[1]) of a natural number n is a positive real number, denoted here that satisfies the following equivalent characterizations:

  • the unique positive real number such that
  • the positive root of the quadratic equation
  • the number
  • the number whose expression as a continued fraction is

Metallic means are generalizations of the golden ratio () and silver ratio (), and share some of their interesting properties. The term "bronze ratio" (), and terms using other metals names (such as copper or nickel), are occasionally used to name subsequent metallic means.[2] [3]

In terms of algebraic number theory, the metallic means are exactly the real quadratic integers that are greater than and have as their norm.

The defining equation of the nth metallic mean is the characteristic equation of a linear recurrence relation of the form It follows that, given such a recurrence the solution can be expressed as

where is the nth metallic mean, and a and b are constants depending only on and Since the inverse of a metallic mean is less than 1, this formula implies that the quotient of two consecutive elements of such a sequence tends to the metallic mean, when k tends to the infinity.

For example, if is the golden ratio. If and the sequence is the Fibonacci sequence, and the above formula is Binet's formula. If one has the Lucas numbers. If the metallic mean is called the silver ratio, and the elements of the sequence starting with and are called the Pell numbers. The third metallic mean is sometimes called the "bronze ratio".

Geometry

If one removes n largest possible squares from a rectangle with ratio length/width equal to the nth metallic mean, one gets a rectangle with the same ratio length/width (in the figures, n is the number of dotted lines).
Golden ratio within the pentagram (φ = red/ green = green/blue = blue/purple) and silver ratio within the octagon.

The defining equation of the nth metallic mean induces the following geometrical interpretation.

Consider a rectangle such that the ratio of its length L to its width W is the nth metallic ratio. If one remove from this rectangle n squares of side length W, one gets a rectangle similar to the original rectangle; that is, a rectangle with the same ratio of the length to the width (see figures).

The nth metallic ratio is the arithmetic mean of the hypothenuse and the shortest leg of a right triangle with side lengths of 2 and n. This results from the value and the Pythagorean theorem.

Some metallic means appear as segments in the figure formed by a regular polygon and its diagonals. This is in particular the case for the golden ratio and the pentagon, and for the silver ratio and the octagon see figures

Powers


Denoting by the metallic mean of m one has

where the numbers are defined recursively by the initial conditions K0 = 0 and K1 = 1, and the recurrence relation

Proof: The equality is immediately true for The recurrence relation implies which makes the equality true for Supposing the equality true up to one has

End of the proof.

One has also [citation needed]

The odd powers of a metallic mean are themselves metallic means. More precisely, if n is an odd natural number, then where is defined by the recurrence relation and the initial conditions and

Proof: Let and The definition of metallic means implies that and Let Since if n is odd, the power is a root of So, it remains to prove that is an integer that satisfies the given recurrence relation. This results from the identity

This completes the proof, given that the initial values are easy to verify.

In particular, one has

and, in general,[citation needed]

where

For even powers, things are more complicate. If n is a positive even integer then[citation needed]

Additionally,[citation needed]

Generalization

One may define the metallic mean of a negative integer n as the positive solution of the equation The metallic mean of n is the multiplicative inverse of the metallic mean of n:

Another generalization consists of changing the defining equation from to . If

is any root of the equation, one has

The silver mean of m is also given by the integral[citation needed]

Another form of the metallic mean is[citation needed]

Numerical values

First metallic means[4][5]
N Ratio Value Name
0 0 + 4/2 1
1 1 + 5/2 1.618033989[a] Golden
2 2 + 8/2 2.414213562[b] Silver
3 3 + 13/2 3.302775638[c] Bronze
4 4 + 20/2 4.236067978[d]
5 5 + 29/2 5.192582404[e]
6 6 + 40/2 6.162277660[f]
7 7 + 53/2 7.140054945[g]
8 8 + 68/2 8.123105626[h]
9 9 + 85/2 9.109772229[i]
10 10+ 104/2 10.099019513[j]

See also

Notes

  1. ^ Sloane, N. J. A. (ed.). "Sequence A001622 (Decimal expansion of golden ratio phi (or tau) = (1 + sqrt(5))/2)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
  2. ^ OEISA014176, Decimal expansion of the silver mean, 1+sqrt(2).
  3. ^ OEISA098316, Decimal expansion of [3, 3, ...] = (3 + sqrt(13))/2.
  4. ^ OEISA098317, Decimal expansion of phi^3 = 2 + sqrt(5).
  5. ^ OEISA098318, Decimal expansion of [5, 5, ...] = (5 + sqrt(29))/2.
  6. ^ OEISA176398, Decimal expansion of 3+sqrt(10).
  7. ^ OEISA176439, Decimal expansion of (7+sqrt(53))/2.
  8. ^ OEISA176458, Decimal expansion of 4+sqrt(17).
  9. ^ OEISA176522, Decimal expansion of (9+sqrt(85))/2.
  10. ^ OEISA176537, Decimal expansion of (10+sqrt(104)/2.

References

  1. ^ M. Baake, U. Grimm (2013) Aperiodic order. Vol. 1. A mathematical invitation. With a foreword by Roger Penrose. Encyclopedia of Mathematics and its Applications, 149. Cambridge University Press, Cambridge, ISBN 978-0-521-86991-1.
  2. ^ de Spinadel, Vera W. (1999). "The metallic means family and multifractal spectra" (PDF). Nonlinear analysis, theory, methods and applications. 36 (6). Elsevier Science: 721–745.
  3. ^ de Spinadel, Vera W. (1998). Williams, Kim (ed.). "The Metallic Means and Design". Nexus II: Architecture and Mathematics. Fucecchio (Florence): Edizioni dell'Erba: 141–157.
  4. ^ Weisstein, Eric W. "Table of Silver means". MathWorld.
  5. ^ "An Introduction to Continued Fractions: The Silver Means", maths.surrey.ac.uk.

Further reading

  • Stakhov, Alekseĭ Petrovich (2009). The Mathematics of Harmony: From Euclid to Contemporary Mathematics and Computer Science, p. 228, 231. World Scientific. ISBN 9789812775832.

External links