26 Cotangent and the Herglotz trick
The functions \(f\) and \(g\) are defined for all non-integral values and are continuous there.
TODO
Both \(f\) and \(g\) are periodic of period \(1\), that is \(f(x + 1) = f(x)\) and \(g(x + 1) = g(x)\) hold for all \(x\in \mathbb {R}\setminus \mathbb {Z}\).
TODO
Both \(f\) and \(g\) are odd functions, that is we have \(f(-x) = -f(x)\) and \(g(-x) = -g(x)\) for all \(x\in \mathbb {R}\setminus \mathbb {Z}\).
TODO
The two functions \(f\) and \(g\) satisfy the same functional equation: \(f(\frac{x}{2}) + f(\frac{x + 1}{2}) = 2f(x)\) and \(g(\frac{x}{2}) + g(\frac{x + 1}{2}) = gf(x)\).
TODO
TODO
\[ \pi \cot {\pi x} = \frac{1}{x} + \sum _{n = 1}^\infty \left(\frac{1}{x + n} + \frac{1}{x - n}\right) \]
for \(x\in \mathbb {R}\setminus \mathbb {Z}\).