Complex Cosine Function is Entire/Proof 1
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Theorem
Let $\cos: \C \to \C$ be the complex cosine function.
Then $\cos$ is entire.
Proof
By the definition of the complex cosine function, $\cos$ admits a power series expansion about $0$:
- $\ds \cos z = \sum_{n \mathop = 0}^\infty \paren {-1}^n \frac {z^{2 n} } {\paren {2 n}!}$
By Complex Function is Entire iff it has Everywhere Convergent Power Series, to show that $\cos$ is entire it suffices to show that this series is everywhere convergent.
From Radius of Convergence from Limit of Sequence: Complex Case, it is sufficient to show that:
- $\ds \lim_{n \mathop \to \infty} \size {\frac {\paren {-1}^{n + 1} } {\paren {2 n + 2}!} \times \frac {\paren {2 n}!} {\paren {-1}^n} } = 0$
We have:
\(\ds \lim_{n \mathop \to \infty} \size {\frac {\paren {-1}^{n + 1} } {\paren {2 n + 2}!} \times \frac {\paren {2 n}!} {\paren {-1}^n} }\) | \(=\) | \(\ds \size {-1} \lim_{n \mathop \to \infty} \size {\frac {\paren {2 n}!} {\paren {2 n + 2} \paren {2 n + 1} \paren {2 n}!} }\) | Definition of Factorial | |||||||||||
\(\ds \) | \(=\) | \(\ds \lim_{n \mathop \to \infty} \paren {\frac 1 {\paren {2 n + 2} \paren {2 n + 1} } }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds 0\) |
Hence the result.
$\blacksquare$