Analysis of information sources in references of the Wikipedia article "Hagiopolitan Octoechos" in English language version.
Raasted, Jørgen (1966). Intonation Formulas and Modal Signatures in Byzantine Musical Manuscripts. Monumenta Musicae Byzantinae, Subsidia. Vol. 7. Copenhagen: E. Munksgaard. Strunk, William Oliver (1942). "The Tonal System of Byzantine Music". The Musical Quarterly. 28 (2): 190–204. doi:10.1093/mq/XXVIII.2.190.The precise nature of the steps within this series remains for the present unknown; for all that we can learn from the Papadike, the step α [prôtos] to β [devteros] may be a whole tone, a half tone, or some other larger or smaller interval. ... If we may assume, however, that the interval α [prôtos] to δ [tetartos] is a perfect fourth–a reasonable assumption, to say the least, for a tetrachordal system based on any other interval is virtually inconceivable–the interval δ [tetartos] to α [prôtos], as the difference between an octave and two fourths, becomes a whole tone and the remaining intervals fall readily into line.