STYX.tex

\RequirePackage{stix}
\documentclass{amsart}
\DeclareMathOperator\bsum{\pmb\sum}
\DeclareMathOperator*\bsm{\pmb\sum}
\begin{document}
With
\[
    \frac1{2+\frac1{3+\cdots}} \quad \cfrac[l]1{2 + \cfrac[l]1{3+\cdots}}
\]

With
\[
    \frac1{\displaystyle 2 + \frac13} \quad \frac1{2 + \frac13} \quad
    \frac1{\scriptstyle 2 + \frac13} \quad \frac1{\scriptscriptstyle 2 + \frac13}
\]

He's funny $\sum \quad \boldsymbol\sum \quad \pmb\sum{i=1}^n \quad \bsum\limits_{i=1}^b \quad \bsm\limits_{i=1}^b$

Bold $\boldsymbol{\mathcal{AMS}}$ {\mathversion{bold}$\mathcal{AMS}$}
\[
    \boldsymbol 5 \quad \mathbf 5 \quad \boldsymbol\alpha \quad
    \boldsymbol\Lambda \quad \boldsymbol{\mathcal A} \quad
    \boldsymbol\to \quad \boldsymbol A \quad
    \mathbf A
\]
And {\mathversion{bold} $a \equiv c \pod\theta$}

And $\boldsymbol\alpha_{\boldsymbol\beta} \quad \boldsymbol\alpha_\beta$

Well $x$ \textit x x $\mathit\Gamma \varGamma \mathcal A \nsubseteq \mathcal a \mathcal b$
 
Or $\mathit{left-side}\ left-side$
\begin{align*}
    a \mathbin\alpha b\\
    a \mathrel\alpha b\\
    a \alpha b
\end{align*}
\begin{align*}
    \sideset{}{_a^b}\prod \text{ and } \sideset{^e}{}\prod\\
    \sideset{}{'}\sum_{\substack{i<10\\j<10}} x_iz_j\\
    \sideset{}{^{'}}\sum_{\substack{i<10\\j<10}} x_iz_j\\
    \sideset{_a^b}{_c^d}{\sum}_{\to}^{\to}
\end{align*}
\begin{align*}
    a \not\in b \quad a \not= b \quad a \notin b \quad a \ne b\\
    a \not\mid b \quad a \nmid b \quad a \mathrel{\not|} b \quad a \mathrel{\not\mid} b
\end{align*}
\begin{align*}
    f(x) \overset{\text{def}}= x^2 - 1\\
    f(x) \overset{\normalfont\text{def}}= x^2 - 1\\
    f(x) \overset{def}= x^2 - 1\\
    f(x) \overset{\mathbf{def}}= x^2 - 1\\
    f(x) \overset{\text{\textbf{def}}}= x^2 - 1\\
    \frac ab \overset u+ \frac cd \overset l+ \frac ef \overset\cdot= 3\\
    \frac ab \overset{\normalfont u}+ \frac cd \overset{\normalfont l}+ \frac ef \overset\cdot= 3
\end{align*}
Overset
\[
    \overset\alpha a \quad
    \underset{\boldsymbol\cdot}X \quad
    \underset\cdot X \quad
    \overset\alpha{a_i} \quad
    {\overset\alpha a}_i
\]
\end{document}