\documentclass{article}

\usepackage{amsmath, amssymb}

\usepackage{bm}

\newcommand{\inprod}[2]{\ensuremath{#1 \cdot #2}}
\newcommand{\dd}[1]{\ensuremath{d#1\,}}
\newcommand{\pr}[1]{\left(#1\right)}
\newcommand{\tr}[1]{\ensuremath{\,{}^{t}\!#1}}
\DeclareMathOperator{\erf}{erf}

\newcommand{\Kst}{\ensuremath{\bm{K}}}
\newcommand{\hyper}{\ensuremath{\bm{\rho}}}

\begin{document}

\begin{equation}
  \label{eq:34}
    p_{\mathrm{cut}}\pr{\bm{a} \mid \hyper, m_{\mathrm{low}}, m_{\mathrm{high}}, \bm{r}}
    =
    \frac{
      2p\pr{\bm{a} \mid \hyper}
      H\pr{\inprod{\bm{r}}{\bm{a}} - m_{\mathrm{low}}}
      H\pr{m_{\mathrm{high}} - \inprod{\bm{r}}{\bm{a}}}
    }{
      \erf{\pr{m_{\mathrm{high}}/\sqrt{2\tr{\bm{r}}\Kst^{-1}\bm{r}}}}
      -
      \erf{\pr{m_{\mathrm{low}}/\sqrt{2\tr{\bm{r}}\Kst^{-1}\bm{r}}}}
    }
\end{equation}
\end{document}

