Mathematical theory of regular temperaments: Difference between revisions

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=== Just intonation subgroups and transversals ===

A relatively concrete approach, but one which is not canonically defined, is to define a [[transversal]] for the temperament by giving generators for a just intonation subgroup which when tempered becomes the notes of the temperament.

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== Geometry of regular temperaments ==

~~Abstract regular temperaments can be identified with~~ {{w|~~rational point~~}}s on an {{w|algebraic variety}} known as a {{w|Grassmannian}}. In particular, if the number of primes in the ''p''-limit is ''n'', and the rank of the temperament is ''r'', then the real Grassmannian '''Gr'''(''r'', ''n'') has points identified with the ''r''-dimensional subspaces of the ''n''-dimensional real vector space '''R'''<sup>''n''</sup>. This has an embedding into a real vector space known as the [[Plücker embedding]], which if the point in question corresponds to a temperament is the wedgie of the temperament. Regular temperaments of rank ''r'' in the ''p''-limit may be defined as rational points on '''Gr'''(''r'', ''n''), though we should note that most of these do not correspond to anything worth much as a temperament. In matrix terms, the real Grassmannian '''Gr'''(''r'', ''n'') can be identified with real symmetric projection matrices with trace ''r''. The rational symmetric projection matrices with trace ''r'' are precisely the Frobenius projections, so under this identification it is clear they represent rational points on '''Gr'''(''r'', ''n''). A rational projection matrix of trace ''r'' which is not symmetric is still a [[tuning map]]; minimax and least squares tunings provide examples of this.

Grassmannians have the structure of a smooth, homogenous {{w|metric space}}, and hence represent a distinctly geometric mathematical object. In the 5-limit, the Grassmannian '''Gr'''(2, 3), consisting of the planes through the origin in three dimensional space, may be identified with the projective plane, and hence 5-limit rank two temperaments may be pictured as points in a projective plane, as below (known as "projective tone space").

See also [[equivalence continuum]] for a description of the space of rank-''r'' temperaments supported by a given temperament, such as a rank-1 temperament, as an algebraic variety.

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 === Just intonation subgroups and transversals ===
 {{Main| Just intonation subgroups | Transversal }}
+{{See also| Gencom }}
 A relatively concrete approach, but one which is not canonically defined, is to define a [[transversal]] for the temperament by giving generators for a just intonation subgroup which when tempered becomes the notes of the temperament.
@@ Line 82: / Line 83: @@
 == Geometry of regular temperaments ==
-Abstract regular temperaments can be identified with {{w|rational point}}s on an {{w|algebraic variety}} known as a {{w|Grassmannian}}. In particular, if the number of primes in the ''p''-limit is ''n'', and the rank of the temperament is ''r'', then the real Grassmannian '''Gr'''(''r'',&nbsp;''n'') has points identified with the ''r''-dimensional subspaces of the ''n''-dimensional real vector space '''R'''<sup>''n''</sup>. This has an embedding into a real vector space known as the [[Plücker embedding]], which if the point in question corresponds to a temperament is the wedgie of the temperament. Regular temperaments of rank ''r'' in the ''p''-limit may be defined as rational points on '''Gr'''(''r'',&nbsp;''n''), though we should note that most of these do not correspond to anything worth much as a temperament. In matrix terms, the real Grassmannian '''Gr'''(''r'',&nbsp;''n'') can be identified with real symmetric projection matrices with trace ''r''. The rational symmetric projection matrices with trace ''r'' are precisely the Frobenius projections, so under this identification it is clear they represent rational points on '''Gr'''(''r'',&nbsp;''n''). A rational projection matrix of trace ''r'' which is not symmetric is still a [[tuning map]]; minimax and least squares tunings provide examples of this.
+{{Main|Plucker coordinates}}
-Grassmannians have the structure of a smooth, homogenous {{w|metric space}}, and hence represent a distinctly geometric mathematical object. In the 5-limit, the Grassmannian '''Gr'''(2,&nbsp;3), consisting of the planes through the origin in three dimensional space, may be identified with the projective plane, and hence 5-limit rank two temperaments may be pictured as points in a projective plane, as below (known as "projective tone space").
 See also [[equivalence continuum]] for a description of the space of rank-''r'' temperaments supported by a given temperament, such as a rank-1 temperament, as an algebraic variety.