Mathematical notations are wrongly displayed

Question

I type a series of mathematical equations in Latex, but output is wrongly displayed (They are highlighted in yellow), how could I fix this problem.

The packages used:

\documentclass[letter,dvips,11pt]{article}
\usepackage{mathrsfs}
\usepackage{oldlfont,epsfig,latexsym}
\usepackage{amsmath}
\usepackage{amsfonts,amssymb,amscd,color}
\usepackage{bm}
\usepackage[flushleft]{threeparttable}
\usepackage{array,booktabs,makecell}

\usepackage{standalone}
\usepackage{tikz}
\usepackage{verbatim}
\usepackage{xcolor}
\usepackage{graphicx}
\usepackage[justification=centering]{caption}

and the corresponding codes in yellow

\begin{aligned}
    \Omega^{i}_{k\mid {k}} &=  \Omega^{i}_{k\mid {k-1}}+ |\mathcal{N}|\sum_{j\in \mathcal{N}} \pi_{L,k}^{i,j} (H_k^j)^T (R_k^j)^{-1}H_k^j  \notag\\
    q_{k\mid {k}}^i &=  q^{i}_{k\mid {k-1}}+ |\mathcal{N}|\sum_{j\in \mathcal{N}} \pi_{L,k}^{i,j} (H_k^j)^T (R_k^j)^{-1} z_k^j
    \end{aligned}

Answer 1

You get this behaviour because you load the package oldlfont which restores the old font behaviour of LaTeX 2.09.

As a result in your example \mathcal doesn't take an argument any more, but acts as a switch (much like \small still does). So in order to keep its effect limited to the N we need to say {\mathcal N} instead of \mathcal{N}.

As far as I can see, oldlfont is really only there to support legacy documents and should not be used in new documents you write. (In in mathrm doesn't change back pdflatex (pre-install of texmaker) egreg writes: "oldlfont [...] is a package to be used only for typesetting documents written with LaTeX2.09, that's been obsolete for more than twenty years.")

You had better drop oldlfont:

\documentclass{article}
\usepackage{mathtools}
\DeclarePairedDelimiter\abs{\lvert}{\rvert}

\begin{document}
\begin{align}
    \Omega^{i}_{k\mid {k}} &=  \Omega^{i}_{k\mid {k-1}}+ \abs{\mathcal{N}}\sum_{j\in \mathcal{N}} \pi_{L,k}^{i,j} (H_k^j)^T (R_k^j)^{-1}H_k^j \\
    q_{k\mid {k}}^i &=  q^{i}_{k\mid {k-1}}+ \abs{\mathcal{N}}\sum_{j\in \mathcal{N}} \pi_{L,k}^{i,j} (H_k^j)^T (R_k^j)^{-1} z_k^j
\end{align}
\end{document}