Neo-Pythagorean philosopher and mathematician who wrote Arithmētikē eisagōgē (Introduction to Arithmetic), an influential treatise on number theory. Considered a standard authority for 1,000 years, the book sets out the elementary theory and properties of numbers and contains the earliest-known Greek multiplication table.

Nicomachus was interested in philosophical questions dealing with whole numbers, the classification of even and odd numbers and their ratios, and wondrous or curious properties of numbers. For example, he was interested in the notion of “perfect numbers,” such as 6, which equals the sum of its proper divisors, and “amicable numbers,” pairs of numbers, such as 220 and 284, whose proper divisors sum to one another. He was not interested, however, in abstract theorems on whole numbers and their proofs, as found in Books VII–IX of Euclid's Elements; contrary to Euclid's approach, Nicomachus would merely give specific numerical examples. A Latin translation of the Arithmētikē by Lucius Apuleius (Apuleius, Lucius) (c. 124–170) is lost, but a version by Ancius Boethius (Boethius, Anicius Manlius Severinus) (c. 470–524) survived and was used in schools up to the Renaissance. Nicomachus also wrote Encheiridion Harmonikēs (“Handbook of Harmony”) on the Pythagorean theory of music (Pythagoreanism) and the two-volume Theologoumena arithmetikēs (“The Theology of Numbers”) on the mystic properties of numbers; only fragments of the latter survive.