\documentclass[12pt]{article}
\usepackage{amsmath,amssymb,amsfonts}
\begin{document}
The two-point correlation function of QED is considered in the worldline formalism. In the position space, it is presented in terms of heat kernel. This leads to introducing the $K_{1}(x_{i})$ functions related to the bulk to boundary propagator of massless scalar field and to revealing the bulk to boundary propagator in the photon polarization operator.
\end{document}