# What is the difference between a singularity and a pole?

From what I could find, a singularity is a point at which an equation, surface, etc., blows up or becomes degenerate.

And a pole of a function is an isolated singular point a of single-valued character of an analytic function $f(z)$ of the complex variable $z$ for which $|f(z)|$ increases without bound when $z$ approaches $a$: $\lim_{z\rightarrow a}f(z) = \infty.$

I really don't fully understand this definition of a pole, like (what is an isolated singular point) and the limit says for $\lim_{z\rightarrow a}f(z) = \infty.$ What is $a$ that $z$ should approach?

$f = 1/(z-1) \ e^{z}$

Can I say $f$ has a singularity at $z = 1$ because we get $1/0$ at that point i.e. blows up and gives $\infty$?

• "isolated" means that there is a neighborhood of that point such that no other singularities are in that neighborhood. $a$ is the point which is a (potential) singularity. All poles are singularities, but not vice versa (there are things called "essential singularities" which are "worse" and "removable singularities" which are "better") Dec 11 '14 at 20:47
• We call a point $a$ a pole of order $n$ of a function f if $\lim_{z\to a}|f(z)|=\infty$ and $\lim_{z\to a}z^nf(z)$ exist and is finite. A pole is a special case of the singularity. Dec 11 '14 at 20:48
• @Frank you surely meant $(z-a)^nf(z)$.
– quid
Dec 11 '14 at 20:52
• @Frank per your definition,since $\lim_{z\rightarrow 1}$ (1$/$(z-1)$) = \infty$. and $\lim_{z\rightarrow 1} ($(z-1)f(z) is finite does this mean $z = 1$ is a pole?. Dec 11 '14 at 21:12
• @quid Of course...thank you for pointing out. Dec 13 '14 at 0:45

One says that $z_0$ is an isolated singularity of $f$ if $f$ is defined in a punctured neighborhood $D\setminus\{z_0\}$ of $z_0$.
One says $z_0$ is a removable singularity of $f$ if there exists a holomorphic function $F(z)$ defined on $D$ which extends $f$.
Suppose $f$ is nonvanishing in a punctured neighborhood $D\setminus\{z_0\}$ of $z_0$. Define $F(z)$ on $D$ by $F(z) = 1/f(z)$ if $z \neq z_0$ and $F(z_0)=0$. Then $z_0$ is a pole of $f$ if $F$ is holomorphic at $z_0$.
A singularity $z_0$ is an essential singularity of $f$ if $z_0$ is neither a pole nor a removable singularity.