# Toronto Math Forum

## MAT334-2018F => MAT334--Tests => Quiz-6 => Topic started by: Victor Ivrii on November 17, 2018, 04:14:52 PM

Title: Q6 TUT 5301
Post by: Victor Ivrii on November 17, 2018, 04:14:52 PM
Locate each of the isolated singularities of the given function $f(z)$ and tell whether it is a removable singularity, a pole, or an essential singularity.
If the singularity is removable, give the value of the function at the point; if the singularity is a pole, give the order of the pole:
$$f(z) =\pi \cot(\pi z).$$
Title: Re: Q6 TUT 5301
Post by: Nikita Dua on November 17, 2018, 04:31:56 PM
My solution
Title: Re: Q6 TUT 5301
Post by: Xinyan Jiang on November 17, 2018, 05:19:25 PM
To begin, $f(z) = \pi \text{cot}(\pi z) = \pi \frac{\text{cos}(\pi z)}{\text{sin}(\pi z)}$

Let the numerator be denoted by $h(z) = \pi \text{cos}(\pi z)$,

and the denominator be $g(z) = \text{sin}(\pi z)$

Then the denominator $g(z) = \text{sin}(\pi z) = 0 \implies z = k, k \in \mathbb{Z}$

Now, $h(k) = \pi \text{cos}(k\pi) = \pi \text{ or } -\pi \neq 0 \implies \text{order = 0}$

$g(k) = \text{sin}(k\pi) = 0$

$g'(z)= \pi \text{cos}(\pi z) \implies g'(k) = \pi \text{cos}(k\pi) = \pi \text{ or } -\pi \neq 0 \implies \text{order = 1}$

$\therefore$ this function has a pole singularity with order of pole = 1 - 0 = 1
Title: Re: Q6 TUT 5301
Post by: Jeffery Mcbride on November 17, 2018, 05:24:26 PM
Why 2k? It should just be k.

$\displaystyle \begin{array}{{>{\displaystyle}l}} f( z) \ =\ \pi \ cot\ ( \pi z)\\ \\ =\frac{\pi cos( \pi z)}{sin( \pi z)} \end{array}$

So clearly, f(z) has a pole when sin($\displaystyle \pi z) \ =\ 0$ which happens whenever z is any integer
This pole is order 1.
Title: Re: Q6 TUT 5301
Post by: Xinyan Jiang on November 17, 2018, 05:34:28 PM
Why 2k? It should just be k.

$\displaystyle \begin{array}{{>{\displaystyle}l}} f( z) \ =\ \pi \ cot\ ( \pi z)\\ \\ =\frac{\pi cos( \pi z)}{sin( \pi z)} \end{array}$

So clearly, f(z) has a pole when sin($\displaystyle \pi z) \ =\ 0$ which happens whenever z is any integer
This pole is order 1.

You are right :( , it was a rudimentary mistake. I have corrected my answer :) thanks
Title: Re: Q6 TUT 5301
Post by: Jeffery Mcbride on November 17, 2018, 05:39:12 PM
No problem!  :)
Title: Re: Q6 TUT 5301
Post by: Victor Ivrii on November 28, 2018, 04:40:47 AM