Residue

${\text{Res}}_{z=z_0}f=a_{-1}:f(z)=\sum _{k=-\infty }^{\infty }{a}_k(z-z_0{)}^k$

• The residue of $f$ at a singularity (Math) $z_0$ is the coefficient $a_{-1}$ for the Laurent Series power series expansion of a function