Tikz 中状态之间的平行边

Question

我有一个马尔可夫链，我想在两个状态之间画两条平行边。如果我向左/向右弯曲边缘，我就可以做到这一点，但我希望它们是直的和平行的。我在网站上看到的所有解决方案要么与弯曲有关，要么代码太复杂（对于看似太简单的东西）。我有很多过渡，所以我需要一些简单而有效的东西。提前谢谢。

下面是边缘重叠的 mwe：

\documentclass{article}  
\usepackage{tikz}
\usetikzlibrary{automata,arrows,positioning}

\begin{document}
\begin{tikzpicture} 
\node[state](a) {$a$};
\node[state,right=of a] (b) {$b$};  
\draw[every loop]  
(a) edge[auto=right] node {$\lambda$} (b)  
(b) edge[auto=right] node {$\mu$} (a);  
\end{tikzpicture}
\end{document}

Answer 1

有几种可能。手动或采用新 (?) 样式。

\documentclass{article}  
\usepackage{multicol}
\usepackage{tikz}
\usetikzlibrary{automata,arrows,positioning,calc}

\begin{document}
\begin{multicols}{2}
\section*{Manual adjustment}
\subsection*{Adjust angles}
\begin{tikzpicture} 
\node[state](a) {$a$};
\node[state,right=of a] (b) {$b$};  
\draw
(a.-10) edge[auto=right,->] node {$\lambda$} (b.190)  
(b.170) edge[auto=right,->] node {$\mu$} (a.10);  
\end{tikzpicture}

\subsection*{Bend}
\begin{tikzpicture} 
\node[state](a) {$a$};
\node[state,right=of a] (b) {$b$};  
\draw
(a) edge[bend right=10,auto=right,->] node {$\lambda$} (b)  
(b) edge[bend right=10,auto=right,->] node {$\mu$} (a);  
\end{tikzpicture}

\vfill\null\columnbreak
\section*{Automatic options}
\subsection*{Auto rotate}
\tikzset{auto rotate/.style={auto=right,->,
to path={let \p1=(\tikztostart),\p2=(\tikztotarget),
\n1={atan2(\y2-\y1,\x2-\x1)},\n2={\n1-10},\n3={\n1+190}
in (\tikztostart.\n2) -- (\tikztotarget.\n3) \tikztonodes}}}
\begin{tikzpicture} 
\node[state](a) {$a$};
\node[state,right=of a] (b) {$b$};  
\node[state,above=of b] (c) {$c$};  
\draw
(a) edge[auto rotate] node {$\lambda$} (b)  
(b) edge[auto rotate] node {$\mu$} (a);  
\draw
(b) edge[auto rotate] node {$\lambda$} (c)  
(c) edge[auto rotate] node {$\mu$} (b);  
\end{tikzpicture}

\subsection*{Auto shift}
\tikzset{auto shift/.style={auto=right,->,
to path={ let \p1=(\tikztostart),\p2=(\tikztotarget),
\n1={atan2(\y2-\y1,\x2-\x1)},\n2={\n1+180}
in ($(\tikztostart.{\n1})!1mm!270:(\tikztotarget.{\n2})$) -- 
($(\tikztotarget.{\n2})!1mm!90:(\tikztostart.{\n1})$) \tikztonodes}}}
\begin{tikzpicture} 
\node[state](a) {$a$};
\node[state,right=of a] (b) {$b$};  
\node[state,above=of b] (c) {$c$};  
\draw
(a) edge[auto shift] node {$\lambda$} (b)  
(b) edge[auto shift] node {$\mu$} (a);  
\draw
(b) edge[auto shift] node {$\lambda$} (c)  
(c) edge[auto shift] node {$\mu$} (b);  
\end{tikzpicture}
\end{multicols}
\end{document}

Answer 2

可以使用tikz-cd包简单地绘制马尔可夫链。例如：

\documentclass[border=3mm]{standalone}
\usepackage{tikz-cd}

\begin{document}
\begin{tikzcd}[cells={nodes={circle, draw=gray,
                             minimum size=3ex, inner sep=1pt, anchor=center}}
              ]
a   \ar[r, bend left, "\lambda"]
&
b   \ar[l, bend left, "\mu"]
\end{tikzcd}

\begin{tikzcd}[cells={nodes={circle, draw=gray,
                             minimum size=3ex, inner sep=1pt, anchor=center}}
              ]
a   \ar[r, shift left, "\lambda"]
&
b   \ar[l, shift left, "\mu"]
\end{tikzcd}
\end{document}