Special Graphs

1. A Directed Acyclic Graph (DAG) is a directed graph that contains no directed cycles.
   
   In a DAG, it is impossible to start at a vertex and follow directed edges back to the same vertex.
   

   This digraph is acyclic because there is no directed path that returns to its starting vertex. Miriam Briskman, CC BY-NC 4.0.

   \documentclass[border=1pt]{standalone}
   \usepackage{tikz}
   \usetikzlibrary{arrows.meta}
   
   \tikzset{
   vertex/.style={draw, circle, very thick, minimum size=1cm},
   edge/.style={very thick, -Triangle}
   }
   
   \begin{document}
   
   \begin{tikzpicture}
   
   \node[vertex] (1) at (0,0) {$1$};
   \node[vertex] (2) at (3,1) {$2$};
   \node[vertex] (3) at (3,-1) {$3$};
   \node[vertex] (4) at (6,0) {$4$};
   
   \draw[edge] (1) -- (2);
   \draw[edge] (1) -- (3);
   \draw[edge] (2) -- (4);
   \draw[edge] (3) -- (4);
   
   \end{tikzpicture}
   
   \end{document}