I know that $\cosh(z)=\cos(iz)$ and $\sinh(z)=-i\sin(iz)$ my question is how can I deduce that $\cosh$ and $\sinh$ are entire? I know that $\cos(x)$ has an infinite radius of convergence and the same goes for $\sin(x)$. If I were to substitute $\cos(iz)$ into the power series for $\cos(x)$ would I be able to show that the radius of convergence is also $\infty$ which would prove that $\cosh(z)$ is entire? To find the derivatives I assume I need to use the Cauchy Riemann equations? Or would I just be able to differentiate the power series?
[Math] Deducing that $\cosh$ and $\sinh$ are entire and calculating the complex derivatives $\sinh’$ and $\cosh’$
complex-analysisderivatives
Best Answer
As mentioned in the comments, the best approach is to use the definitions of the two functions. The product rule tells us that the product of two entire functions is entire. Similarly, the sum rule tells us that the sum of two entire functions is entire and the chain rule tells us that the composition of two entire functions is entire. These three rules say not only what forms the functions derivatives take, but also the fact that they are differentiable (up to some caveats that don't apply here)
Those three facts allow you to conclude that $\sinh$ and $\cosh$ are entire based on their definition in terms of $e^x$, since $e^x$ is an entire function.