Ratio metallica
Ratio metallica (gen. Rationis metallicae) signatur ratio  (cum numero n naturale). n-ima ratio metallica definitur major radix aequationis x2 - nx - 1 = 0 quadraticae. Cifra est una at prima appellatur ratio aurea, secunda ratio argentea, tertia ratio aerea.

Bibliographia
 Stakhov, Alekseĭ Petrovich (2009). The Mathematics of Harmony: From Euclid to Contemporary Mathematics and Computer Science, p. 228, 231. World Scientific. ISBN 9789812775832.

Metalla
Notiones mathematicae