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Hi I found a finite summation of cotangents as $$\sum_{k=0}^{n-1}\cot^2(z + \frac{k\pi}{n}) = n^2 - n + n^2 \cot^2(n z), \quad n > 0$$ in the URL bellow

http://functions.wolfram.com/ElementaryFunctions/Cot/introductions/Cot/ShowAll.html

Can anyone introduce a book or reference which includes the above summation? I want to see the proof and study more on them.

Thanks in advance.

zahra
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Here is the reference that you may find useful:

Cvijović, Djurdje. "Summation formulae for finite cotangent sums." Applied Mathematics and Computation 215.3 (2009): 1135-1140.

Pavel
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