# Superparticular ratio

Short description: Ratio of two consecutive integers
Just diatonic semitone on C: 16/15 = 15 + 1/15 = 1 + 1/15

In mathematics, a superparticular ratio, also called a superparticular number or epimoric ratio, is the ratio of two consecutive integer numbers.

More particularly, the ratio takes the form:

$\displaystyle{ \frac{n + 1}{n} = 1 + \frac{1}{n} }$ where n is a positive integer.

Thus:

A superparticular number is when a great number contains a lesser number, to which it is compared, and at the same time one part of it. For example, when 3 and 2 are compared, they contain 2, plus the 3 has another 1, which is half of two. When 3 and 4 are compared, they each contain a 3, and the 4 has another 1, which is a third part of 3. Again, when 5, and 4 are compared, they contain the number 4, and the 5 has another 1, which is the fourth part of the number 4, etc.
—Throop (2006), [1]

Superparticular ratios were written about by Nicomachus in his treatise Introduction to Arithmetic. Although these numbers have applications in modern pure mathematics, the areas of study that most frequently refer to the superparticular ratios by this name are music theory[2] and the history of mathematics.[3]

## Mathematical properties

As Leonhard Euler observed, the superparticular numbers (including also the multiply superparticular ratios, numbers formed by adding an integer other than one to a unit fraction) are exactly the rational numbers whose continued fraction terminates after two terms. The numbers whose continued fraction terminates in one term are the integers, while the remaining numbers, with three or more terms in their continued fractions, are superpartient.[4]

$\displaystyle{ \prod_{n=1}^{\infty} \left(\frac{2n}{2n-1} \cdot \frac{2n}{2n+1}\right) = \frac{2}{1} \cdot \frac{2}{3} \cdot \frac{4}{3} \cdot \frac{4}{5} \cdot \frac{6}{5} \cdot \frac{6}{7} \cdots = \frac{4}{3}\cdot\frac{16}{15}\cdot\frac{36}{35}\cdots=2\cdot\frac{8}{9}\cdot\frac{24}{25}\cdot\frac{48}{49}\cdots=\frac{\pi}{2} }$

represents the irrational number π in several ways as a product of superparticular ratios and their inverses. It is also possible to convert the Leibniz formula for π into an Euler product of superparticular ratios in which each term has a prime number as its numerator and the nearest multiple of four as its denominator:[5]

$\displaystyle{ \frac{\pi}{4} = \frac{3}{4} \cdot \frac{5}{4} \cdot \frac{7}{8} \cdot \frac{11}{12} \cdot \frac{13}{12} \cdot\frac{17}{16}\cdots }$

In graph theory, superparticular numbers (or rather, their reciprocals, 1/2, 2/3, 3/4, etc.) arise via the Erdős–Stone theorem as the possible values of the upper density of an infinite graph.[6]

## Other applications

In the study of harmony, many musical intervals can be expressed as a superparticular ratio (for example, due to octave equivalency, the ninth harmonic, 9/1, may be expressed as a superparticular ratio, 9/8). Indeed, whether a ratio was superparticular was the most important criterion in Ptolemy's formulation of musical harmony.[7] In this application, Størmer's theorem can be used to list all possible superparticular numbers for a given limit; that is, all ratios of this type in which both the numerator and denominator are smooth numbers.[2]

These ratios are also important in visual harmony. Aspect ratios of 4:3 and 3:2 are common in digital photography,[8] and aspect ratios of 7:6 and 5:4 are used in medium format and large format photography respectively.[9]

## Ratio names and related intervals

Every pair of adjacent positive integers represent a superparticular ratio, and similarly every pair of adjacent harmonics in the harmonic series (music) represent a superparticular ratio. Many individual superparticular ratios have their own names, either in historical mathematics or in music theory. These include the following:

Examples
Ratio Cents Name/musical interval Ben Johnston
notation above C
Audio
2:1 1200 duplex:[lower-alpha 1] octave C' File:Perfect octave on C.mid
3:2 701.96 sesquialterum:[lower-alpha 1] perfect fifth G File:Just perfect fifth on C.mid
4:3 498.04 sesquitertium:[lower-alpha 1] perfect fourth F File:Just perfect fourth on C.mid
5:4 386.31 sesquiquartum:[lower-alpha 1] major third E File:Just major third on C.mid
6:5 315.64 sesquiquintum:[lower-alpha 1] minor third E File:Just minor third on C.mid
7:6 266.87 septimal minor third E File:Septimal minor third on C.mid
8:7 231.17 septimal major second D- File:Septimal major second on C.mid
9:8 203.91 sesquioctavum:[lower-alpha 1] major second D File:Major second on C.mid
10:9 182.40 sesquinona:[lower-alpha 1] minor tone D- File:Minor tone on C.mid
11:10 165.00 greater undecimal neutral second D- File:Greater undecimal neutral second on C.mid
12:11 150.64 lesser undecimal neutral second D File:Lesser undecimal neutral second on C.mid
15:14 119.44 septimal diatonic semitone C File:Septimal diatonic semitone on C.mid
16:15 111.73 just diatonic semitone D- File:Just diatonic semitone on C.mid
17:16 104.96 minor diatonic semitone C File:Minor diatonic semitone on C.mid
21:20 84.47 septimal chromatic semitone D File:Septimal chromatic semitone on C.mid
25:24 70.67 just chromatic semitone C File:Just chromatic semitone on C.mid
28:27 62.96 septimal third-tone D- File:Septimal third-tone on C.mid
32:31 54.96 31st subharmonic,
inferior quarter tone
D- File:Thirty-first subharmonic on C.mid
49:48 35.70 septimal diesis D File:Septimal diesis on C.mid
50:49 34.98 septimal sixth-tone B- File:Septimal sixth-tone on C.mid
64:63 27.26 septimal comma,
63rd subharmonic
C- File:Septimal comma on C.mid
81:80 21.51 syntonic comma C+ File:Syntonic comma on C.mid
126:125 13.79 septimal semicomma D File:Septimal semicomma on C.mid
128:127 13.58 127th subharmonic File:127th subharmonic on C.mid
225:224 7.71 septimal kleisma B File:Septimal kleisma on C.mid
256:255 6.78 255th subharmonic D- File:255th subharmonic on C.mid
4375:4374 0.40 ragisma C- File:Ragisma on C.mid

The root of some of these terms comes from Latin sesqui- "one and a half" (from semis "a half" and -que "and") describing the ratio 3:2.

## Notes

1. Ancient name

## Citations

1. Throop, Priscilla (2006). Isidore of Seville's Etymologies: Complete English Translation, Volume 1, p. III.6.12, n. 7. ISBN:978-1-4116-6523-1.
2. Halsey, G. D.; Hewitt, Edwin (1972). "More on the superparticular ratios in music". American Mathematical Monthly 79 (10): 1096–1100. doi:10.2307/2317424.
3. Robson, Eleanor; Stedall, Jacqueline (2008), The Oxford Handbook of the History of Mathematics, Oxford University Press, ISBN 9780191607448 . On pp. 123–124 the book discusses the classification of ratios into various types including the superparticular ratios, and the tradition by which this classification was handed down from Nichomachus to Boethius, Campanus, Oresme, and Clavius.
4. Leonhard Euler; translated into English by Myra F. Wyman and Bostwick F. Wyman (1985), "An essay on continued fractions", Mathematical Systems Theory 18: 295–328, doi:10.1007/bf01699475 . See in particular p. 304.
5. Debnath, Lokenath (2010), The Legacy of Leonhard Euler: A Tricentennial Tribute, World Scientific, p. 214, ISBN 9781848165267 .
6. Erdős, P.; Stone, A. H. (1946). "On the structure of linear graphs". Bulletin of the American Mathematical Society 52 (12): 1087–1091. doi:10.1090/S0002-9904-1946-08715-7.
7. Barbour, James Murray (2004), Tuning and Temperament: A Historical Survey, Courier Dover Publications, p. 23, ISBN 9780486434063 .
8. Ang, Tom (2011), Digital Photography Essentials, Penguin, p. 107, ISBN 9780756685263 . Ang also notes the 16:9 (widescreen) aspect ratio as another common choice for digital photography, but unlike 4:3 and 3:2 this ratio is not superparticular.
9. The 7:6 medium format aspect ratio is one of several ratios possible using medium-format 120 film, and the 5:4 ratio is achieved by two common sizes for large format film, 4×5 inches and 8×10 inches. See e.g. Schaub, George (1999), How to Photograph the Outdoors in Black and White, How to Photograph Series, 9, Stackpole Books, p. 43, ISBN 9780811724500 .