[tuning-math] Four 10-note, 7-limit JI scales

If we take (10/9)^2 (15/14)^2 (16/15)^2 (21/20)^3 = 2 as scale steps, and
simplify the scale-finding problem by assuming 4/3 and 3/2 both belong to
the scale, we obtain four scales, the third and fourth of which are the
inverted forms of the first and second. A version of the major/minor
transformation, exchanging 10/9 with 16/15, which is equivalent to saying
2-->2, 3-->3, 5-->24/5, 7-->168/25, exchanges the first and second, as
well as the third and fourth. The first "decaa", and fourth, "decad", are
major versions, having two major tetrads and a minor tetrad, while
"decab" and "decac" have two minor and one major tetrad. In any system
where 50/49~1 the exchange transform sends tetrads to tetrads and can be
considered major/minor. In 22-et in particular, each scale becomes the
symmetrical decatonic. All of the scales have 23 intervals, 17 triads and
3 tetrads.

! decad.scl
! [15/14, 10/9, 21/20, 16/15, 15/14, 21/20, 10/9, 15/14, 16/15, 21/20]
inversion of decab
10
!
15/14
25/21
5/4
4/3
10/7
3/2
5/3
25/14
40/21
2/1

! decab.scl
! [21/20, 16/15, 15/14, 10/9, 21/20, 15/14, 16/15, 21/20, 10/9, 15/14]
(10/9) <==> (16/15) transform of decaa
10
!
21/20
28/25
6/5
4/3
7/5
3/2
8/5
42/25
28/15
2/1

! decac.scl
! [15/14, 16/15, 21/20, 10/9, 15/14, 21/20, 16/15, 15/14, 10/9, 21/20]
inversion of decaa
10
!
15/14
8/7
6/5
4/3
10/7
3/2
8/5
12/7
40/21
2/1

! decad.scl
! [15/14, 10/9, 21/20, 16/15, 15/14, 21/20, 10/9, 15/14, 16/15, 21/20]
inversion of decab
10
!
15/14
25/21
5/4
4/3
10/7
3/2
5/3
25/14
40/21
2/1



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