11. For which real numbers $\lambda$, the inequality $\frac{1}{2}\left(e^{x}+\right.$ $\left.e^{-\mathrm{x}}\right) \leqslant e^{\lambda \mathrm{x}^{2}}$ holds for all real numbers $x$.
$\left(\lambda \geqslant \frac{1}{2}\right)$