Unit function

In number theory, the unit function is a completely multiplicative function on the positive integers defined as:

${\displaystyle \varepsilon (n)={\begin{cases}1,&{\mbox{if }}n=1\\0,&{\mbox{if }}n\neq 1\end{cases}}}$

It is called the unit function because it is the identity element for Dirichlet convolution.¹

It may be described as the "indicator function of 1" within the set of positive integers. It is also written as ${\displaystyle u(n)}$ (not to be confused with ${\displaystyle \mu (n)}$, which generally denotes the Möbius function).