at (2.7,0) {\ldots};

\node[inner sep=1pt, circle, fill=black](a1a2) at (0,1) {};
\node[inner sep=1pt, circle, fill=black](a2a3) at (1,1) {};
\node[inner sep=1pt, circle, fill=black](a3a4) at (2,1) {};
\node[inner sep=1pt, circle, fill=black](a4ak) at (4,1) {};
\node[inner sep=1pt, circle, fill=black](a1ak) at (5,1) {};
\node at (2.6,1) {\ldots};
\node at (2.9,0.6) {$\ddots$};

\draw[-]  (a1)--(a1a2); \draw[-] (a1)--(2.6, 0.45); \draw[-] (3.2, 0.65)--(a1ak);
\draw[-] (a2)--(a1a2); \draw[-] (a2)--(a2a3);
\draw[-] (a3)--(a2a3); \draw[-] (a3)--(a3a4);
\draw[-] (ak)--(a1ak);
\draw[-] (a4)--(a4ak);
\draw[-] (ak)--(a4ak);
\draw[-] (a3a4)--(2.4,0.7);
\draw[-] (a4)--(3.5,0.3);



\node[anchor = east]  at (a1) {\small{$a_1$}};
\node[anchor =  east]  at (a2) {\small{$a_2$}};
\node[anchor = east]  at (a3) {\small{$a_3$}};
\node[anchor = east]  at (a4) {\small{$a_{n-1}$}};
\node[anchor = east]  at (ak) {\small{$a_n$}};

\node[anchor = east]  at (a1a2) {\small{$a_1a_2$}};
\node[anchor = south]  at (a1ak) {\small{$a_1a_n$}};
\node[anchor =  east]  at (a2a3) {\small{$a_2a_3$}};
\node[anchor = east]  at (a3a4) {\small{$a_3a_4$}};
\node[anchor = east]  at (a4ak) {\small{$a_{n-1}a_n$}};

\end{tikzpicture}$$

It is clear that this Hasse Diagram can be redrawn as: $$\begin{tikzpicture}

\node[regular polygon,regular polygon sides=13,minimum size=5cm,draw] (a){};
\foreach \x in {9,...,13}{\node[circle,fill,inner sep=1pt] at (a.corner \x) {};}
\foreach \x in {1,...,5}{\node[circle,fill,inner sep=1pt] at (a.corner \x) {};}

\draw [line width=1.1cm, white] (1.1,-2) -- (-2.3,-2);
\node at (-1.71,-1.55) {$\ddots$};
\node at (0.85,-2.17) {$\dots$};
\node[anchor = south]  at (a.corner 1) {\small{$a_1$}};
\node[anchor = east]  at (a.corner 2) {\small{$a_1a_n$}};
\node[anchor = 