Metallic mean

Metallic means
0:	0 + √4/2	1
1:	1 + √5/2	1.618033989
2:	2 + √8/2	2.414213562
3:	3 + √13/2	3.302775638
4:	4 + √20/2	4.236067978
5:	5 + √29/2	5.192582404
6:	6 + √40/2	6.162277660
7:	7 + √53/2	7.140054945
8:	8 + √68/2	8.123105626
9:	9 + √85/2	9.109772229
⋮
n:	n + √4 + n2/2

Golden ratio within the pentagram and silver ratio within the octagon.

The metallic means or silver means (also ratios or constants) of the successive natural numbers are the continued fractions

$n + \cfrac{1}{n+\cfrac{1} {n+\cfrac{1} {n+\cfrac{1} {n+\ddots\,}}}} = [n;n,n,n,n,\dots] = \frac{1}{2}\left(n+\sqrt{n^2+4}\right).$

The golden ratio (1.618...) is the metallic mean between 1 and 2, while the silver ratio (2.414...) is the metallic mean between 2 and 3. The term "bronze ratio" (3.303...), or terms using other names of metals (copper, nickel), are occasionally used to name subsequent metallic means. The values of the first ten metallic means are shown at right. Notice that each metallic mean is a root of the simple quadratic equation

$x^2 - nx = 1,\,$ where n is any positive natural number.

As the golden ratio is connected to the pentagon (first diagonal/side), the silver ratio is connected to the octagon (first diagonal/side). As the golden ratio is connected to the Fibonacci numbers, the silver ratio is connected to the Pell numbers, and the bronze ratio is connected to  A006190. Each Fibonacci number is the sum of the previous number times one plus the number before that, each Pell number is the sum of the previous number times two and the one before that, and each "bronze Fibonacci number" is the sum of the previous number times three plus the number before that. Taking successive Fibonacci numbers as ratios, these ratios approach the golden mean, the Pell number ratios approach the silver mean, and the "bronze Fibonacci number" ratios approach the bronze mean.

Properties

File:Gold, silver, and bronze rectangles.svg

If one removes the largest possible square from the end of a gold rectangle one is left with a gold rectangle. If one removes two from a silver, one is left with a silver. If one removes three from a bronze, one is left with a bronze.

120px

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

These properties are valid only for integers m, for nonintegers the properties are similar but slightly different.

The above property for the powers of the silver ratio is a consequence of a property of the powers of silver means. For the silver mean S of m, the property can be generalized as

$\!\ S_{m}^n = K_n S_m + K_{(n-1)}$

where

$\!\ K_n = mK_{(n-1)} + K_{(n-2)}.$

Using the initial conditions K₀ = 1 and K₁ = m, this recurrence relation becomes

$\!\ K_n = \frac{1}{\sqrt{m^2 + 4}} {(S_m^{n+1} - {(m-S_m)}^{n+1})}.$

The powers of silver means have other interesting properties:

If n is a positive even integer:

$\!\ {S_m^n - \lfloor S_m^n \rfloor} = 1 - S_m^{-n}.$

Additionally,

$\!\ {1 \over {S_m^4 - \lfloor S_m^4 \rfloor}} + \lfloor S_m^4 - 1 \rfloor = S_{(m^4 + 4m^2 + 1)}$

$\!\ {1 \over {S_m^6 - \lfloor S_m^6 \rfloor }} + \lfloor S_m^6 - 1 \rfloor = S_{(m^6 + 6m^4 + 9m^2 +1)}.$

File:Golden Triangle.svg

A golden triangle. The ratio a:b is equivalent to the golden ratio φ. In a silver triangle this would be equivalent to δ_S.

Also,

$\!\ S_m^3 = S_{(m^3 + 3m)}$

$\!\ S_m^5 = S_{(m^5 + 5m^3 + 5m)}$

$\!\ S_m^7 = S_{(m^7 + 7m^5 + 14m^3 + 7m)}$

$\!\ S_m^9 = S_{(m^9 + 9m^7 + 27m^5 + 30m^3 + 9m)}$

$\!\ S_m^{11} = S_{(m^{11} + 11m^9 + 44m^7 + 77m^5 + 55m^3 + 11m)}.$

In general:

$\!\ S_m^{2n+1} = S_{\sum_{k=0}^n {{2n+1} \over {2k+1}} {{n+k} \choose {2k}} m^{2k+1}}.$

The silver mean S of m also has the property that

$\!\ 1/S_m = S_m - m$

meaning that the inverse of a silver mean has the same decimal part as the corresponding silver mean.

$\!\ S_m = a + b$

where a is the integer part of S and b is the decimal part of S, then the following property is true:

$\!\ S_m^2 = a^2 + mb + 1.$

Because (for all m greater than 0), the integer part of S_m = m, a = m. For m > 1, we then have

$\!\ S_m^2 = ma + mb + 1$

$\!\ S_m^2 = m(a+b) + 1$

$\!\ S_m^2 = m(S_m) + 1.$

Therefore the silver mean of m is a solution of the equation

$\!\ x^2 - mx - 1 = 0.$

It may also be useful to note that the silver mean S of −m is the inverse of the silver mean S of m

$\!\ 1/S_m = S_{(-m)} = S_m - m.$

Another interesting result can be obtained by slightly changing the formula of the silver mean. If we consider a number

$\!\ \frac{1}{2}\left(n+\sqrt{n^2+4c}\right) = R$

then the following properties are true:

$\!\ R - \lfloor R \rfloor = c/R$ if c is real,

$\!\ \left({1 \over R}\right)c = R - \lfloor \operatorname{Re}(R) \rfloor$ if c is a multiple of i.

The silver mean of m is also given by the integral

$S_m = \int_0^m {\left( {x \over {2\sqrt{x^2+4}}} + {{m+2} \over {2m}} \right)} \, dx.$

See also

• Constant
• Mean
• Ratio
• Plastic number

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

• Stakhov, Alexey. "The Mathematics of Harmony: Clarifying the Origins and Development of Mathematics", PeaceFromHarmony.org.
• Cristina-Elena Hrețcanu and Mircea Crasmareanu (2013). "Metallic Structures on Riemannian Manifolds", Revista de la Unión Matemática Argentina.
• Rakočević, Miloje M. "Further Generalization of Golden Mean in Relation to Euler's 'Divine' Equation", Arxiv.org.