diagram.tex

\documentclass{article}

\usepackage{tikz-cd}

%change style of arrows
\tikzcdset{arrow style=math font}

% circle
\newcommand*\cir[1]{\tikz[baseline=(char.base)]{
    \node[shape=circle,draw,inner sep=2.5pt] (char) {#1};}}


% square
\newcommand*\squ[1]{\tikz[baseline=(char.base)]{
    \node[shape=rectangle,draw,inner sep=4pt] (char) {#1};}}

% empty space
\newcommand{\pha}{\phantom{a}}

\begin{document}

\begin{equation}
\begin{tikzcd}[row sep = 2em]
  & & \cir{\scalebox{0.7}{start}} \ar[dll] \ar[dl] \ar[d] \ar[drr]& & \\
  \cir{1} \ar[drr]& \cir{2} \ar[dr] & \cir{3} \ar[d] & \hspace{-1em}\cdot\cdot\cdot & \cir{p} \ar[dll] \\
  & & \cir{\scalebox{0.7}{p+1}} & &
\end{tikzcd}
\end{equation}

\end{document}