Question about font size of a subscript

Question

I am working on a formulae in a notes. Here is part of my code,

\begin{equation*}
         \phi(x) := \left\{
                    \begin{aligned} 
                         &\chi_{K_n}(\bar{x})\cdot\frac{\psi_n(x)}{\tilde{\psi}_n(\bar{x})}, & \text{ if } \bar{x}\in K_n \\
                         &0                                                                  & \text{ otherwise}     
                    \end{aligned}
                    \right.
     \end{equation*}

The problem is that I need a subcript of a subscript in \chi_{K_n}, but \chi is a small letter, K is a capital letter. With a small subscript n on the right, it looks like \chi times K_n, is there a way to make \chi bigger or make the subscript K smaller? Thanks.

By the way, can I post math formulae here like I do in mathstakexchange?

Answer 1

This is not really great in respect to mathematical syntax, but you could just set it on one layer below. Say, add one more subscript command. I reduced the kerning a bit, as it did look too wide in my eyes.

% arara: pdflatex

\documentclass{article}
\usepackage{mathtools}

\begin{document}
\begin{equation*}
    \phi(x) \coloneqq
    \begin{dcases*} % only write the "d" if you want the fraction in display style
        \chi_{_{K_n}}\mkern-2mu(\bar{x})\cdot\frac{\psi_n(x)}{\tilde{\psi}_n(\bar{x})}, & if $\bar{x}\in K_n$ \\
        0                                                                     & otherwise     
    \end{dcases*}
\end{equation*} 
\end{document}

---

However, it would be preferable if you use an upper-case chi $X$ or a lower-case k.

---

Update:

I collected all mentioned versions and made a screen-shot. You will have to decide for yourself.

\begin{equation*}
\chi_{K_n} % your approach
\chi{_{_{K_n}}} % my approach
\mathrm{X}_{K_n} % upper-case chi
\chi^{}_{k_n} % lower-case k
\chi{\scriptstyle_{K_n}} % honeste_vivere's approach
\chi^{}_{K_{n}} % egreg in first comment
\chi_{\scriptscriptstyle K_{n}} % egreg in second comment
\end{equation*}

Answer 2

Yes, within the subscript you can use something like the \scriptstyle command to change the font size of the subscript or superscript. Here is an example:

\chi_{K}

is the way you were doing it before, but now try:

\chi{\scriptstyle_{K}}

This should make the subscripts smaller.

Answer 3

Here are some methods for lowering the subscript. Making the subscript in the size proper for second level ones is not an option, in my opinion.

My preferred one would be either the second or the third in the upper row; the big cases uses the fourth method (with \shiftdown).

\documentclass{article}
\usepackage{mathtools}

\newcommand{\shiftdown}{\vrule height 1.8ex width 0pt}

\begin{document}
\begin{equation*}
\chi_{K_n} \chi^{}_{K_n} \chi^{\mathstrut}_{K_n} \chi_{\shiftdown K_n}
\end{equation*}

\begin{equation*}
\phi(x) \coloneqq
\begin{dcases*}
\chi_{\shiftdown K_n}(\bar{x})\frac{\psi_n(x)}{\tilde{\psi}_n(\bar{x})},
   & if $\bar{x}\in K_n$ \\
0,
   & otherwise
\end{dcases*}
\end{equation*}

\end{document}