Question about font size of a subscript

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?

• to post output crop an image of the result and then add an image using the editor interface. (mathjax is not used on this site) – David Carlisle Apr 28 '15 at 12:49
• It's sufficient to use \chi^{}_{K_{n}}, which will move the subscript down. – egreg Apr 28 '15 at 13:46
• For your last question: No. You can see Why doesn't maths render as maths? for details. – Paul Gessler Apr 28 '15 at 14:34

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}}} % 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*}


• This is really awful, IMHO. A bit less awful would be \chi_{\scriptscriptstyle K_n}, but it's still the wrong approach. Suggesting cases (or variations thereof) is surely good, instead. – egreg Apr 28 '15 at 13:48
• @FangyangTian You are welcome. If one of the answers is a good solution for you, please accept it as answer. Like this, we will get the post closed. Instead of writing "Thanks" in comments, we tend to upvote the answers. – LaRiFaRi Apr 29 '15 at 14:47

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.

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}