Just intonation subgroup
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By a just intonation subgroup is meant a [[http://en.wikipedia.org/wiki/Free_abelian_group|group]] generated by a finite set of positive rational numbers via arbitrary multiplications and divisions. Any such group will be contained in a [[Harmonic Limit|p-limit]] group for some minimal choice of prime p, which is the prime limit of the subgroup. It is only when the group in question is not the entire p-limit group that we have a just intonation subgroup in the strict sense. Such subgroups come in two flavors: finite [[http://en.wikipedia.org/wiki/Index_of_a_subgroup|index]] and infinite index, where intuitively speaking the index measures the relative size of the subgroup within the entire p-limit group. For example, the subgroups generated by 4 and 3, by 2 and 9, and by 4 and 6 all have index 2 in the full 3-limit (Pythagorean) group. Half of the 3-limit intervals will belong to any one of them, and half will not, and all three groups are distinct. On the other hand, the group generated by 2, 3, and 7 is of infinite index in the full 7-limit group, which is generated by 2, 3, 5 and 7. A canonical naming system for just intonation subgroups is to give a [[Normal lists|normal interval list]] for the generators of the group, which will also show the [[http://en.wikipedia.org/wiki/Rank_of_an_abelian_group|rank]] of the group by the number of generators in the list. Below we give some of the more interesting subgroup systems. If a scale is given with the system, it means the subgroup is generated by the notes of the scale. ====7-limit==== [2, 3, 7] Ets: 5, 31, 36, 135, 571 [2, 5, 7] Ets: 6, 25, 31, 171, 239, 379, 410, 789 [2, 3, 7/5] Ets: 10, 29, 31, 41, 70, 171, 241, 412 [2, 5/3, 7] Ets: 12, 15, 42, 57, 270, 327 [2, 5/3, 7/3] Ets: 27, 68, 72, 99, 171, 517 ====11-limit==== [2, 3, 11] Ets: 7, 15, 17, 24, 159, 494, 518, 653 [2, 5, 11] Ets: 6, 7, 9, 13, 15, 22, 37, 87, 320 [2, 7, 11] Ets: 6, 9, 11, 20, 26, 135, 161, 296
Original HTML content:
<html><head><title>Just intonation subgroups</title></head><body>By a just intonation subgroup is meant a <a class="wiki_link_ext" href="http://en.wikipedia.org/wiki/Free_abelian_group" rel="nofollow">group</a> generated by a finite set of positive rational numbers via arbitrary multiplications and divisions. Any such group will be contained in a <a class="wiki_link" href="/Harmonic%20Limit">p-limit</a> group for some minimal choice of prime p, which is the prime limit of the subgroup. <br /> <br /> It is only when the group in question is not the entire p-limit group that we have a just intonation subgroup in the strict sense. Such subgroups come in two flavors: finite <a class="wiki_link_ext" href="http://en.wikipedia.org/wiki/Index_of_a_subgroup" rel="nofollow">index</a> and infinite index, where intuitively speaking the index measures the relative size of the subgroup within the entire p-limit group. For example, the subgroups generated by 4 and 3, by 2 and 9, and by 4 and 6 all have index 2 in the full 3-limit (Pythagorean) group. Half of the 3-limit intervals will belong to any one of them, and half will not, and all three groups are distinct. On the other hand, the group generated by 2, 3, and 7 is of infinite index in the full 7-limit group, which is generated by 2, 3, 5 and 7.<br /> <br /> A canonical naming system for just intonation subgroups is to give a <a class="wiki_link" href="/Normal%20lists">normal interval list</a> for the generators of the group, which will also show the <a class="wiki_link_ext" href="http://en.wikipedia.org/wiki/Rank_of_an_abelian_group" rel="nofollow">rank</a> of the group by the number of generators in the list. Below we give some of the more interesting subgroup systems. If a scale is given with the system, it means the subgroup is generated by the notes of the scale.<br /> <br /> <!-- ws:start:WikiTextHeadingRule:0:<h4> --><h4 id="toc0"><a name="x---7-limit"></a><!-- ws:end:WikiTextHeadingRule:0 -->7-limit</h4> [2, 3, 7]<br /> Ets: 5, 31, 36, 135, 571<br /> <br /> [2, 5, 7]<br /> Ets: 6, 25, 31, 171, 239, 379, 410, 789<br /> <br /> [2, 3, 7/5]<br /> Ets: 10, 29, 31, 41, 70, 171, 241, 412<br /> <br /> [2, 5/3, 7]<br /> Ets: 12, 15, 42, 57, 270, 327<br /> <br /> [2, 5/3, 7/3]<br /> Ets: 27, 68, 72, 99, 171, 517<br /> <br /> <!-- ws:start:WikiTextHeadingRule:2:<h4> --><h4 id="toc1"><a name="x---11-limit"></a><!-- ws:end:WikiTextHeadingRule:2 -->11-limit</h4> <br /> [2, 3, 11]<br /> Ets: 7, 15, 17, 24, 159, 494, 518, 653<br /> <br /> [2, 5, 11]<br /> Ets: 6, 7, 9, 13, 15, 22, 37, 87, 320<br /> <br /> [2, 7, 11]<br /> Ets: 6, 9, 11, 20, 26, 135, 161, 296</body></html>