If lim x → + ∞ f ( x ) = c {\displaystyle \lim _{x\to +\infty }f(x)=c} or lim x → − ∞ f ( x ) = c {\displaystyle \lim _{x\to -\infty }f(x)=c} , then the horizontal line y = c {\displaystyle y=c} is a horizontal asymptote of the function.
