Argument Principle
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Theorem
Let $f$ be a function meromorphic in the interior of some simply connected region $D$.
Let $f$ be holomorphic with no zeroes on the boundary of $D$.
Let $N$ denote the number of zeroes of $f$ in the interior of $D$, counted up to multiplicity.
Let $P$ denote the number of poles of $f$ in the interior of $D$, counted up to order.
Then:
- $\displaystyle N - P = \frac 1 {2\pi i} \oint_D \frac { f'\left({z}\right) } { f\left({z}\right) } \, \mathrm d z$
Proof
Let $n_1, n_2, n_3 \ldots n_N$ denote the zeroes of $f$, and $p_1, p_2, p_3 \ldots p_P$ denote its poles. Then, there exists a function $g$ nonzero and holomorphic on the boundary and on the interior of $D$ such that:
- $\displaystyle f\left({z}\right) = \frac { \prod_{k \mathop = 1}^N \left( { z - n_k } \right) } { \prod_{j \mathop = 1}^P \left( { z - p_j } \right) } g\left({z}\right)$
Taking the logarithmic derivative:
| \(\displaystyle \mathcal L \left( { f\left( {z} \right) } \right)\) | \(=\) | \(\displaystyle \frac { f'\left({z}\right) } { f\left({z}\right) }\) | $\quad$ Definition of Logarithmic Derivative of Meromorphic Function | $\quad$ | |||||||||
| \(\displaystyle \) | \(=\) | \(\displaystyle \mathcal L \left( { \prod_{k \mathop = 1}^N \left( { z - n_k } \right) } \right) - \mathcal L \left( { \prod_{j \mathop = 1}^P \left( { z - p_j } \right) } \right) + \mathcal L \left( { g\left({z}\right) } \right)\) | $\quad$ | $\quad$ | |||||||||
| \(\displaystyle \) | \(=\) | \(\displaystyle \sum_{k \mathop = 1}^N \frac 1 {z - n_k} - \sum_{j \mathop = 1}^P \frac 1 {z - p_j} + \frac { g'\left({z}\right) } { g\left({z}\right) }\) | $\quad$ | $\quad$ |
Then:
| \(\displaystyle \oint_D \frac { f'\left({z}\right) } { f\left({z}\right) } \, \mathrm d z\) | \(=\) | \(\displaystyle \oint_D \left( \sum_{k \mathop = 1}^N \frac 1 {z - n_k} - \sum_{j \mathop = 1}^P \frac 1 {z - p_j} + \frac { g'\left({z}\right) } { g\left({z}\right) } \right) \, \mathrm d z\) | $\quad$ | $\quad$ | |||||||||
| \(\displaystyle \) | \(=\) | \(\displaystyle \sum_{k \mathop = 1}^N \oint_D \frac 1 {z - n_k} \, \mathrm d z - \sum_{j \mathop =1}^P \oint_D \frac 1 {z - p_j} \, \mathrm d z + \oint_D \frac { g'\left({z}\right) } { g\left({z}\right) } \, \mathrm d z\) | $\quad$ Linear Combination of Contour Integrals | $\quad$ | |||||||||
| \(\displaystyle \) | \(=\) | \(\displaystyle \sum_{k=1}^N 2\pi i - \sum_{j \mathop = 1}^P 2\pi i + \oint_D \frac{ g'\left({z}\right) } { g\left({z}\right) } \, \mathrm d z\) | $\quad$ | $\quad$ | |||||||||
| \(\displaystyle \) | \(=\) | \(\displaystyle 2\pi i \left( {N - P} \right) + \oint_D \frac{ g'\left({z}\right) } { g\left({z}\right) } \, \mathrm d z\) | $\quad$ | $\quad$ |
By our construction of $g$, $\frac {g'} g$ is holomorphic on and inside $D$ so, by the Cauchy Integral Theorem, the integral is equal to zero. We therefore have:
- $\displaystyle \oint_D \frac { f'\left({z}\right) } { f\left({z}\right) } \, \mathrm d z = 2\pi i \left( {N - P} \right)$
Hence:
- $\displaystyle N - P = \frac 1 {2\pi i} \oint_D \frac { f'\left({z}\right) } { f\left({z}\right) } \, \mathrm d z$
$\blacksquare$
