207zpi
207 Zeta Peak Index (abbreviated 207zpi) is the equal-step tuning system derived from the 207th peak of the [Riemann Zeta Function].
Theory
207zpi is the strongest zeta peak corresponding to 45edo, and serves as a [stretched-octave] version thereof (2/1 ≈ 1204.289¢). It substantially improves on 45edo's harmonic accuracy, with no non-powers of 2/1 below 16/1 exceeding 7.5¢ error.
Crucially, due to the octave stretch, the 207zpi patent vals of 9/1 and 15/1 have the same values as their "b-vals" (the second best approximation of a Just Intonation interval in a tuning system) in 45edo ([k]=142 and k=175 steps for both systems); i.e. the already-sharp direct approximations of those harmonics in 45edo (k=143 and k=176), which are not found within its flattone diatonic scale, are "pushed out of the way" by the octave stretch within 207zpi. This means that the direct approximations of 9/1, 15/1 are now mapped to the diatonic scale, though this is not the case for their octave-reduced counterparts of 9/8 and 15/8 (k=8 and k=41 in both systems).
Approximation of Harmonics
| Harmonic | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Absolute error (¢) | 4.289 | -1.855 | 8.577 | -3.069 | 2.433 | 3.182 | 12.866 | -3.710 | 1.220 | -3.213 | 6.722 | 1.959 | 7.471 | -4.924 | -9.607 |