$$x\operatorname{ctg} x = \sum\limits_{n = 0}^{ + \infty } {{{\left( { - 1} \right)}^n}{B_{2n}}\frac{{{2^{2n}}}}{{\left( {2n} \right)!}}{x^{2n}},{\text{ }}\left| x \right| < \pi } $$