See also slides Node as information and edges as relationship between data Directed acyclic graph (DAG) Application: Merkle DAG undirected. (\mathcal{N}, \mathcal{E}) \mathcal{N}: set of vertices \mathcal{E}: set of undirected edges: \mathcal{E} \subseteq \mathcal{N} \times \mathcal{N} path is a sequence of nodes and edges connect two nodes.