Re-Expressing the Digamma I was reading some articles on the digamma function, and I was wondering if anyone knows how to express the digamma function $\psi^{(0)}(n)$ in terms of a trigonometric function or a logarithmic function. Does anyone know its Fourier Series representation? 
 A: The following formulas may be inserted into the formula given by @Lucian
$$H_n=\sum_{k=1}^n\frac{1}{k} 
$$
$$
H_n-log\left(n\right)=1-\sum_{k=1}^{\infty}\left(\sum_{i=nk+1}^{n(k+1)}\frac{1}{i}-\frac{1}{k+1}\right)
$$
https://math.stackexchange.com/a/1602945/134791
$$
\gamma= \sum_{n=1}^\infty \left(\frac{2}{n}-\sum_{j=n(n-1)+1}^{n(n+1)} \frac{1}{j}\right)=\sum_{k=1}^\infty \left(\frac{2}{k}-\sum_{j=k(k-1)+1}^{k(k+1)} \frac{1}{j}\right)
$$
https://math.stackexchange.com/a/1591256/134791
$$\psi_0\left(n+1\right)=H_n-\gamma$$ 
$$=\sum_{k=1}^n\frac{1}{k}-\sum_{k=1}^\infty \left(\frac{2}{k}-\sum_{j=k(k-1)+1}^{k(k+1)} \frac{1}{j}\right)$$
In the complex plane outside negative integers (http://people.math.sfu.ca/~cbm/aands/page_259.htm)
$$\psi(z+1)=-\gamma+\sum_{n=1}^\infty\frac{z}{n(n+z)}=-\gamma+\sum_{n=1}^\infty\left(\frac{1}{n}-\frac{1}{n+z}\right)$$
$$=-\sum_{n=1}^\infty \left(\frac{2}{n}-\sum_{j=n(n-1)+1}^{n(n+1)} \frac{1}{j}\right)+\sum_{n=1}^\infty\left(\frac{1}{n}-\frac{1}{n+z}\right)$$
Finally,
$$\psi(z+1)=\sum_{n=1}^\infty \left(-\frac{1}{n}-\frac{1}{n+z} +\sum_{j=n(n-1)+1}^{n(n+1)}\frac{1}{j} \right), z \neq -1,-2,-3,...$$
