Formula_14894

\[\varphi(x)=\left(\frac{1}{2 \pi}\right)^{-3 / 2} \int \frac{d \boldsymbol{k}}{\sqrt{2 k_{0}}}\left[a^{+}(\boldsymbol{k}) e^{i k x}+a^{-}(\boldsymbol{k}) e^{-i k x}\right] .\]