Substack with varying font size

Question

I've been trying to replicate the following figure from https://en.wikipedia.org/wiki/Cauchy%E2%80%93Riemann_equations:

More specifically, I am interested in the varying font size in the limit. My current implementation is

\[
  \lim_{\substack{h \to 0 \\ 
                 {\scriptscriptstyle h \in \mathbb{C}}}}
\]

which works, but I was wondering if there is a more direct way to implement this, without invoking the \scriptscitpstyle or font scaling.

Answer 1

Split the input at \\ and then deliver it using \\ \scriptscriptstyle for the items after the first one.

\documentclass{article}
\usepackage{amsmath,amssymb}

\ExplSyntaxOn
\NewDocumentCommand{\subsubstack}{m}
 {
  \seq_set_split:Nnn \l_tmpa_tl { \\ } {#1}
  \substack{\seq_map_indexed_function:NN \l_tmpa_tl \__danya_subsubstack:nn}
 }
\cs_new_protected:Nn \__danya_subsubstack:nn
 {
  \int_compare:nTF { #1=1 } {#2} { \\ \scriptscriptstyle #2 }
 }
\ExplSyntaxOff

\begin{document}

\[
  \lim_{\subsubstack{h \to 0 \\ h \in \mathbb{C}}}
  \ne
  \lim_{\subsubstack{h \to 0 \\ h \in \mathbb{R} \\ h>0 }}
\]

\end{document}

Answer 2

If you look at the code in the Wikipedia article linked in your question, it uses \underset to typeset the limit conditions:

\documentclass{article}
\usepackage{amsmath,amssymb}

\begin{document}
\[
  \lim_{\underset{h\in\mathbb{C}}{h\to 0}} % etc.
\]
\end{document}