2の12乗根
Barbour, J. M. (1933). “A Sixteenth Century Approximation for π”. American Mathematical Monthly 40 (2): 69–73. doi:10.2307/2300937. JSTOR 2300937.
Ellis, Alexander; Helmholtz, Hermann (1954). On the Sensations of Tone. Dover Publications. ISBN 0-486-60753-4
Partch, Harry (1974). Genesis of a Music. Da Capo Press. ISBN 0-306-80106-X