align環境下でマーカーみたいな矢印と注釈をつけたいなって思ってて
ただtikzでやるにしてもどうやったらいいのかわからん
欲を張ると注釈は二段がいいな pic.twitter.com/2YalW8BpII— たにこー (@M_brass_inst) June 16, 2025
\documentclass{article}
\usepackage{amsmath,tikz}
\usetikzlibrary{tikzmark,quotes}
\begin{document}
\begin{align}
\delta S[q] &= S[q+\varepsilon] - S[q] \notag \\
&= \int_{t_1}^{t_2} \tikzmarknode{1}{dt}
\left\{ L(q+\varepsilon,\dot{q}
+\dot{\varepsilon},t) - L(q,\dot{q},t)
\right\} \notag \\[11ex]
&= \int_{t_1}^{t_2} \tikzmarknode{2}{dt}
\left\{ L + \varepsilon
\frac{\partial L}{\partial q} + \dot{\varepsilon}
\frac{\partial L}{\partial \dot{q}}
+ O(\varepsilon^2,\dot{\varepsilon}^2) - L \right\}
\end{align}
\begin{tikzpicture}[overlay, remember picture]
\draw[-stealth, shorten >=1ex, shorten <=1ex]
(1.south) to ["$
\begin{aligned}
\varepsilon &> 0 \\
t_1 &\not= t_2
\end{aligned}
$"] (2.north);
\end{tikzpicture}
\end{document}