
$$Q\left(x\right)=\frac{1}{\sqrt{2\pi}}\underset{x}{\overset{\infty}{\int}}{e}^{{u}^{2}/2}du\text{.}$$ 
$$Q\left(x\right)=\frac{1}{2}\mathrm{erfc}\left(\frac{x}{\sqrt{2}}\right)$$ 
$$\left\epsilon \right\simeq \left\frac{x{e}^{{x}^{2}/2}}{\sqrt{2\pi}Q\left(x\right)}\delta \right\text{.}$$ 
$$\leftE\right\simeq \left\frac{x{e}^{{x}^{2}/2}}{\sqrt{2\pi}}\delta \right$$ 