答案1
画出这样的东西很容易,因为 Ti钾Z 解析其表达式。
\documentclass[tikz,border=3mm]{standalone}
\begin{document}
\begin{tikzpicture}
\draw[stealth-stealth] (0,5) |- (8,0) node[right]{$n$};
\pgfmathsetseed{7}
\draw plot[variable=\x,samples at={0.3,0.6,...,2.4,3.3,3.7,...,8},only marks,mark=*,
mark options={blue}] (\x,{2+2*sin(360*rnd)*exp(-0.25*\x)});
\draw[densely dashed] (0,2) -- (8,2) node[right]{$a$};
\draw(3,2) circle[x radius=0.4,y radius=1];
\fill[red] (2.7,3.1) circle[radius=2pt];
\fill[green!70!black] (3,1.1) circle[radius=2pt];
\draw[green!70!black,latex-] (3,1) -- ++(0,-0.3) node[below]{$a_{N_0}$};
\draw (3,0.1) -- (3,-0.1) node[below]{$N_0$};
\foreach \X in {+,-}
{\draw (3,2\X 1) -- (8,2\X 1) node[right]{$a\X\varepsilon$};}
\end{tikzpicture}
\end{document}