Superparticular numbers, also called epimoric ratios, are ratios of the form
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 apart 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), 
Superparticular numbers were written about by Nicomachus in his treatise "Introduction to Arithmetic". As well as having applications within mathematics, they are useful in the study of music theory and visual harmony.
In the study of harmony, many musical intervals can be expressed as a superparticular ratio. 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.
These ratios are also important in visual harmony. Most flags of the world's countries have a ratio of 3:2 between their length and height. Aspect ratios of 4:3 and 3:2 are common in digital photography, and aspect ratios of 7:6 and 5:4 are used in medium format and large format photography respectively.
|Ratio||Name||Related musical interval||Audio|
|3:2||sesquialterum||perfect fifth||Play (help·info)|
|4:3||sesquitertium||perfect fourth||Play (help·info)|
|5:4||sesquiquartum||major third||Play (help·info)|
|6:5||sesquiquintum||minor third||Play (help·info)|
|7:6||septimal minor third||Play (help·info)|
|9:8||sesquioctavum||major second||Play (help·info)|
|10:9||sesquinona||minor tone||Play (help·info)|
|16:15||just diatonic semitone||Play (help·info)|
|25:24||just chromatic semitone||Play (help·info)|
|81:80||syntonic comma||Play (help·info)|
|126:125||septimal semicomma||Play (help·info)|
The root of some of these terms comes from Latin sesqui- "one and a half" (from semis "a half" + -que "and") describing the ratio 3:2.
See also 
- 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.
- Halsey, G. D.; Hewitt, Edwin (1972). "More on the superparticular ratios in music". American Mathematical Monthly (Mathematical Association of America) 79 (10): 1096–1100. doi:10.2307/2317424. JSTOR 2317424. MR0313189.
- 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.
- An Arithmetical Rubric by Siemen Terpstra, about the application of superparticular numbers to harmony.[dead link]
- Superparticular numbers applied to construct pentatonic scales by David Canright.
- De Institutione Arithmetica, liber II by Anicius Manlius Severinus Boethius
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