$$\frac{1}{{\cosh x}} = \sum\limits_{k = 0}^{ + \infty } {\frac{{{E_{2k}}{x^{2k}}}}{{\left( {2k} \right)!}}} ;$$ $$\frac{1}{{\cos x}} = \sum\limits_{k = 0}^{ + \infty } {\frac{{{{\left( { - 1} \right)}^k}{E_{2k}}{x^{2k}}}}{{\left( {2k} \right)!}}} ;$$ \[{E_{2k}}{\text{ - Euler numbers}}\]