Interval variety

The interval variety of an interval class in a scale is the number of different interval qualities available for that interval class. For example, the interval class "fifth" in the diatonic scale has interval variety 2, because there are two sizes of fifths in that scale: 6 perfect fifths and 1 diminished fifth.

The concept of interval variety can be applied to all interval classes of a scale at once. Here are some such properties:

• Highest interval variety (see also maximum variety)
• Mean interval variety
• Median interval variety
• Lowest interval variety

In addition, strict variety scales, such as single-period MOS scales and trivalent scales, have the same interval variety for all interval classes (except the unison, which always trivially has interval variety 1).

Note: A standard academic counterpart to the xen term variety is the abelian complexity function of a word: a function ρ^ab : N -> N where ρ^ab(n) is the number of distinct "sizes" that length-n subwords can have in a word.

Facts

Abstractly SV4 scale patterns

Abstractly SV4 scale patterns (patterns that are SV4 for any choice of distinct cent values for the four steps):

• 4 notes: 0123
• 5 notes: 01023
• 6 notes: none
• 7 notes: 0123210, 0102013, and 0103102 (The last two patterns are a chiral pair fixing concrete sizes for the steps.)
• 8 notes: 00100232 and 01212103
• 9 notes: none
• 10 notes: 0010020302 and 0102103012
• 11 notes: none
• 12 notes: none
• 13 notes: none

(Note that abstract SV4-ness implies that a scale pattern is primitive, or single-period.)

Open questions

• Why are (abstractly) SV4 scale patterns seemingly so rare?
  • Conjecture: There are only finitely many SV4 circular words of finite length.
  • Related may be the following conjecture: For a sufficiently long ternary linear word, there exists k > 1 such that the interval class of k-steps has at least 3 sizes and the interval class of (k − 1)-steps also has at least 3 sizes.