# Nome (mathematics)

In mathematics, specifically the theory of elliptic functions, the nome is a special function and is given by

${\displaystyle q=e^{-{\frac {\pi K'}{K}}}=e^{\frac {{\rm {i}}\pi \omega _{2}}{\omega _{1}}}=e^{{\rm {i}}\pi \tau }\,}$[clarification needed]

where K and iK′ are the quarter periods, and ω1 and ω2 are the fundamental pair of periods. Notationally, the quarter periods K and iK′ are usually used only in the context of the Jacobian elliptic functions, whereas the half-periods ω1 and ω2 are usually used only in the context of Weierstrass elliptic functions. Some authors, notably Apostol, use ω1 and ω2 to denote whole periods rather than half-periods.

The nome is frequently used as a value with which elliptic functions and modular forms can be described; on the other hand, it can also be thought of as function, because the quarter periods are functions of the elliptic modulus. This ambiguity occurs because for real values of the elliptic modulus, the quarter periods and thus the nome are uniquely determined.

The function τ = iK ′/K = ω21 is sometimes called the half-period ratio because it is the ratio of the two half-periods ω1 and ω2 of an elliptic function.

The complementary nome q1 is given by

${\displaystyle q_{1}=e^{-{\frac {\pi K}{K'}}}.\,}$

Some sources however use convention ${\displaystyle q=e^{{2{\rm {i}}}\pi \tau }}$ or ${\displaystyle q=e^{{2{\rm {i}}}\pi z}}$.

See the articles on quarter period and elliptic integrals for additional definitions and relations on the nome.