What is a digital signal? What mathematical method is used to express it?

(N/A) digital signal is a signal that can take only two discrete values,typically represented as $0$ (low) and $1$ (high). These values correspond to specific voltage levels in electronic circuits.
The mathematical method used to express digital signals is known as $Boolean$ $Algebra$. It is a branch of algebra in which the values of the variables are the truth values $true$ and $false$,usually denoted by $1$ and $0$ respectively.