Sqrt(2187/2048)
sqrt(2187/2048), the half-apotome, or semiaugmented unison, is a radical interval of about 57 cents, in the sqrt(2).sqrt(3) subgroup or dicot (ploidacot) more precisely. It is a quartertone-like interval; differing from the smaller quartertone sqrt(256/243) by half a pythagorean comma. It is reached by +7 hemififths octave reduced, with a fractional monzo of [-11/2 7/2⟩.
In temperaments
The half-apotome can be made equal to many rational intervals, most notably, to 33/32, tempering out the rastma, or much more accurately to 1323/1280, tempering out the breedsma.
It is most often represented in scores by a half-sharp, . However, this does not mean C = D always; the latter is the smaller quartertone sqrt(256/243). This equality only holds true if the pythagorean comma is tempered out too.