Reduce spacing between letters in equations

Question

Summary: Unhappy with spacing between 'J' and 'a' in my equation

The paper I am looking at has the Jakob number as follows:

I try to repeat the same formula using

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{equation}
        Ja = \text{Ja} = \frac{c_{pl} \left( T_w - T_{sat} \right)}{h_{fg}}
\end{equation}
\end{document}

but my equation ends up looking like:

and after adding suggested solutions:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{equation}
    \mathit{Ja} = Ja = \mathrm{Ja} = \text{Ja} = \frac{c_{pl} \left( T_w - T_{sat} \right)}{h_{fg}}
\end{equation}
\end{document}

Take a look at the Jakob number, the 'Ja' part between the two pictures. What should I do to achieve the first Jakob number letter spacing, without having to change the font?

Answer 1

You want to use \mathit{Ja}, so text italic rather than math italic is used. The same treatment should probably be done for “pl”, “sat” and “fg”, which don't seem like product of variables.

\documentclass[twocolumn]{article} % twocolumn just for a smaller picture
\usepackage{amsmath}

\newcommand{\Jakob}{\mathit{Ja}}
\newcommand{\tsub}[1]{\mathit{#1}}% for textual subscripts

\begin{document}

\subsection*{Bad formula}
\begin{equation}
Ja = \frac{c_{pl} \left( T_w - T_{sat} \right)}{h_{fg}}
\end{equation}

\subsection*{Good formula}

\begin{equation}
\Jakob = \frac{c_{\tsub{pl}}( T_w - T_{\tsub{sat}} )}{h_{\tsub{fg}}}
\end{equation}

\end{document}

You can compare. Note that \left and \right only add unwanted space.

Using \tsub for those subscripts is important, because you might be told (or decide yourself) that such subscripts should be upright and changing the definition would be enough. With

\newcommand{\tsub}[1]{\mathrm{#1}}

the output would become

Kerning

If you really want to push the “a” nearer to the “J”, use \mspace and not \kern.

With \mspace:

\documentclass[twocolumn]{article}
\usepackage{amsmath}

\newcommand{\Jakob}{\mathit{J\mspace{-4mu}a}}
\newcommand{\tsub}[1]{\mathit{#1}}% for textual subscripts

\begin{document}

\section*{Jakob Number $\Jakob$}

\begin{equation}
\Jakob = \frac{c_{\tsub{pl}}( T_w - T_{\tsub{sat}} )}{h_{\tsub{fg}}}
\end{equation}

\end{document}

With \kern:

\documentclass[twocolumn]{article}
\usepackage{amsmath}

\newcommand{\Jakob}{\mathit{J\kern-2pt a}}
\newcommand{\tsub}[1]{\mathit{#1}}% for textual subscripts

\begin{document}

\section*{Jakob Number $\Jakob$}

\begin{equation}
\Jakob = \frac{c_{\tsub{pl}}( T_w - T_{\tsub{sat}} )}{h_{\tsub{fg}}}
\end{equation}

\end{document}

You can easily see that the kerning in the section title is wrong.

Font choice

The picture seems to use Cambria and here an additional kerning doesn't seem necessary.

\documentclass[twocolumn]{article}
\usepackage{amsmath}
\usepackage{unicode-math}

\setmainfont{Cambria}
\setmathfont{Cambria Math}

\newcommand{\Jakob}{\mathit{Ja}}
\newcommand{\tsub}[1]{\mathit{#1}}% for textual subscripts

\begin{document}

\subsection*{Bad formula}
\begin{equation}
Ja = \frac{c_{pl} \left( T_w - T_{sat} \right)}{h_{fg}}
\end{equation}

\subsection*{Good formula}

\begin{equation}
\Jakob = \frac{c_{\tsub{pl}}( T_w - T_{\tsub{sat}} )}{h_{\tsub{fg}}}
\end{equation}

\end{document}

Compile with LuaLaTeX

Answer 2

If not given any additional information, TeX typesets $Ja$ as the product of two distinct variables named J and a.

If that's not your intention -- say, because Ja is supposed to be a variable name in its own right (short for "Jakob number", I gather) -- and assuming that you're ok with using italic lettering to denote variable names, I suggest you write $\mathit{Ja}$, so as to achieve a slightly tighter spacing, or kerning, between the letters J and a.

A further thought: If you're not quite satisfied with the slightly-tighter kerning delivered by $\mathit{Ja}$, I suggest you create your own macro to impose your preferred kerning amount. E.g., you could define

\newcommand{\Ja}{\mathit{J\mkern-3.5mu a}}

in the preamble and then write $\Ja$ in the body of the document. (I suppose that any kerning amount between -3mu and -4mu will look ok.)

\documentclass{article}
\newcommand{\Ja}{\mathit{J\kern-3.5mu a}} % use in math mode only

\begin{document}
$Ja$ vs.\ $\mathit{Ja}$ vs.\ $\Ja$
\end{document}