Solution
= Solution
An <isolated singularity> at $a$ means that $f$ is holomorphic in some punctured disk $0<|z-a|<r$, while $a$ itself is an excluded point or a point where the given function is not holomorphic. There are no other singularities arbitrarily close to $a$. By <Laurent's theorem>, the function has a unique <Laurent series> on that punctured disk:
$$
f(z)=\sum_{j=-\infty}^{\infty}c_j(z-a)^j.
$$
The negative-power terms form its <principal part of a Laurent series>.