\documentclass[tikz,border=5pt]{standalone}
\usepackage{amssymb,amsthm}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[xscale=1.45,yscale=.05,every node/.style={circle,minimum size=4ex,inner sep=1pt},font=\small]
\node[circle,draw] (1) at (0,0) {$1$};
\node[circle] (a) at (1,10) {$\lambda^{-1}$};
\node[circle,draw] (b) at (1,-10) {$\lambda^{-2}$};
\node[circle,draw] (aa) at (2,20) {$\lambda^{-3}$};
\node[circle] (ab) at (2,0) {$\lambda^{-2}$};
\node[circle] (ba) at (2,-20) {$\lambda^{-2}$};
\node[circle,draw] (aab) at (3,30) {$\lambda^{-3}$};
\node[circle,draw] (aba) at (3,10) {$\lambda^{-2}$};
\node[circle,draw] (baa) at (3,-10) {$\lambda^{-3}$};
\node[circle] (bab) at (3,-30) {$\lambda^{-4}$};
\node[circle] (aaba) at (4,40) {$\lambda^{-3}$};
\node[circle] (abaa) at (4,20) {$\lambda^{-3}$};
\node[circle,draw] (abab) at (4,0) {$\lambda^{-4}$};
\node[circle,draw] (baab) at (4,-20) {$\lambda^{-3}$};
\node[circle,draw] (baba) at (4,-40) {$\lambda^{-4}$};
\node[circle,draw] (aabaa) at (5,50) {$\lambda^{-5}$};
\node[circle] (aabab) at (5,30) {$\lambda^{-4}$};
\node[circle] (abaab) at (5,10) {$\lambda^{-3}$};
\node[circle] (ababa) at (5,-10) {$\lambda^{-4}$};
\node[circle] (baaba) at (5,-30) {$\lambda^{-3}$};
\node[circle] (babaa) at (5,-50) {$\lambda^{-4}$};
\node[circle,draw] (aabaab) at (6,60) {$\lambda^{-5}$};
\node[circle,draw] (aababa) at (6,40) {$\lambda^{-4}$};
\node[circle,draw] (abaaba) at (6,20) {$\lambda^{-3}$};
\node[circle,draw] (ababaa) at (6,0) {$\lambda^{-4}$};
\node[circle,draw] (baabaa) at (6,-20) {$\lambda^{-5}$};
\node[circle] (baabab) at (6,-40) {$\lambda^{-4}$};
\node[circle] (babaab) at (6,-60) {$\lambda^{-4}$};
\node[circle] (aabaaba) at (7,70) {$\lambda^{-5}$};
\node[circle] (aababaa) at (7,50) {$\lambda^{-4}$};
\node[circle] (abaabaa) at (7,30) {$\lambda^{-5}$};
\node[circle,draw] (abaabab) at (7,10) {$\lambda^{-4}$};
\node[circle,draw] (ababaab) at (7,-10) {$\lambda^{-4}$};
\node[circle,draw] (baabaab) at (7,-30) {$\lambda^{-5}$};
\node[circle,draw] (baababa) at (7,-50) {$\lambda^{-4}$};
\node[circle,draw] (babaaba) at (7,-70) {$\lambda^{-4}$};
\node[circle] (aabaabab) at (8,80) {$\lambda^{-5}$};
\node[circle] (aababaab) at (8,60) {$\lambda^{-4}$};
\node[circle] (abaabaab) at (8,40) {$\lambda^{-5}$};
\node[circle] (abaababa) at (8,20) {$\lambda^{-4}$};
\node[circle] (ababaaba) at (8,0) {$\lambda^{-4}$};
\node[circle] (baabaaba) at (8,-20) {$\lambda^{-5}$};
\node[circle] (baababaa) at (8,-40) {$\lambda^{-4}$};
\node[circle] (babaabaa) at (8,-60) {$\lambda^{-5}$};
\node[circle,draw] (babaabab) at (8,-80) {$\lambda^{-6}$};
\draw[above,node font=\small] (1) edge node{$a$} (a);
\draw[below,node font=\small] (1) edge node{$b$} (b);
\draw[above,node font=\small] (a) edge node{$a$} (aa);
\draw[below,node font=\small] (a) edge node{$b$} (ab);
\draw[below,node font=\small] (b) edge node{$a$} (ba);
\draw[above,node font=\small] (aa) edge node{$b$} (aab);
\draw[above,node font=\small] (ab) edge node{$a$} (aba);
\draw[above,node font=\small] (ba) edge node{$a$} (baa);
\draw[below,node font=\small] (ba) edge node{$b$} (bab);
\draw[above,node font=\small] (aab) edge node{$a$} (aaba);
\draw[above,node font=\small] (aba) edge node{$a$} (abaa);
\draw[below,node font=\small] (aba) edge node{$b$} (abab);
\draw[below,node font=\small] (baa) edge node{$b$} (baab);
\draw[below,node font=\small] (bab) edge node{$a$} (baba);
\draw[above,node font=\small] (aaba) edge node{$a$} (aabaa);
\draw[below,node font=\small] (aaba) edge node{$b$} (aabab);
\draw[below,node font=\small] (abaa) edge node{$b$} (abaab);
\draw[below,node font=\small] (abab) edge node{$a$} (ababa);
\draw[below,node font=\small] (baab) edge node{$a$} (baaba);
\draw[below,node font=\small] (baba) edge node{$a$} (babaa);
\draw[above,node font=\small] (aabaa) edge node{$b$} (aabaab);
\draw[above,node font=\small] (aabab) edge node{$a$} (aababa);
\draw[above,node font=\small] (abaab) edge node{$a$} (abaaba);
\draw[above,node font=\small] (ababa) edge node{$a$} (ababaa);
\draw[above,node font=\small] (baaba) edge node{$a$} (baabaa);
\draw[below,node font=\small] (baaba) edge node{$b$} (baabab);
\draw[below,node font=\small] (babaa) edge node{$b$} (babaab);
\draw[above,node font=\small] (aabaab) edge node{$a$} (aabaaba) ;
\draw[above,node font=\small] (aababa) edge node{$a$} (aababaa) ;
\draw[above,node font=\small] (abaaba) edge node{$a$} (abaabaa) ;
\draw[below,node font=\small] (abaaba) edge node{$b$} (abaabab) ;
\draw[below,node font=\small] (ababaa) edge node{$b$} (ababaab) ;
\draw[below,node font=\small] (baabaa) edge node{$b$} (baabaab) ;
\draw[below,node font=\small] (baabab) edge node{$a$} (baababa) ;
\draw[below,node font=\small] (babaab) edge node{$a$} (babaaba) ;
\draw[above,node font=\small] (aabaaba) edge node{$b$} (aabaabab);
\draw[above,node font=\small] (aababaa) edge node{$b$} (aababaab);
\draw[above,node font=\small] (abaabaa) edge node{$b$} (abaabaab);
\draw[above,node font=\small] (abaabab) edge node{$a$} (abaababa);
\draw[above,node font=\small] (ababaab) edge node{$a$} (ababaaba);
\draw[above,node font=\small] (baabaab) edge node{$a$} (baabaaba);
\draw[above,node font=\small] (baababa) edge node{$a$} (baababaa);
\draw[above,node font=\small] (babaaba) edge node{$a$} (babaabaa);
\draw[below,node font=\small] (babaaba) edge node{$b$} (babaabab);
\end{tikzpicture}
\end{document}