[Tex/LaTex] Relative positioning of node with TikZ Positioning


I am trying to replicate an automata with tikz and was wondering if there is a keyword that would allow be to mimic the position of the states with relative positioning without a "dummy" node. Below is my current solution with a blank node and an image of what I am trying to replicate the state positions of:

\usepackage{fancyhdr, float}

\usepackage{amsmath, amssymb, amsthm}



\begin{tikzpicture}[shorten >=1pt,node distance=2cm,on grid,auto] 
    \node[state,initial] (A_2) {$A_2$};
    \node (Z) [right=of A_2] {};
    \node[state,accepting] (B_2) [right=of Z] {$B_2$};
    \node[state] (C_2) [below=of Z] {$C_2$};
        (A_2) edge [bend left=15] node {$\varepsilon$} (B_2)
    edge [swap] node {$\a$} (C_2)
        (B_2) edge [bend left=15] node {$\a$} (A_2)
        (C_2) edge [loop right] node {$\b$} ()
    edge [swap] node {$\a,\b$} (B_2);

enter image description here

Best Answer

I changed \a,\b to \tta,\ttb, because it's not a good idea to overwrite existing macros. Their original definition (in article) is

> \a=macro:
#1->\expandafter \@changed@cmd \csname \string #1\endcsname \relax .

> \b=macro:
->\OT1-cmd \b \OT1\b .

To get the gray background use state/.append style={fill=black!10}. The arrow can be mimicked by >=stealth.

\begin{tikzpicture}[>=stealth,auto,state/.append style={fill=black!10}] 
    \node[state,initial] (A2) {$A_2$};
    \node[state,below right=of A2] (C2) {$C_2$};
    \node[state,accepting,above right=of C2] (B2) {$B_2$};
    \draw (A2) edge[->,bend left=15] node {$\varepsilon$} (B2);
    \draw (B2) edge[->,bend left=15] node {$\tta$} (A2);
    \draw (C2) edge[<-] node {$\tta$} (A2);
    \draw (B2) edge[<-] node {$\tta,\ttb$} (C2);
    \draw (C2) edge [loop right] node {$\ttb$} (C2);

enter image description here