> [!tldr] [[Hierarchy]], [[Polyhiearchy]], [[DAG|Directed Acyclic Graph]]s, etc A note on the language of [[Graph Theory]] - which is a branch[^1] of [[Discrete Mathematics]]. ## [[Graph]]-related Terms - **Basic terms** - **Edge** - aka "arrow" or "line" - **Node** - aka vertex - **Directionality** - **Directed** - where nodes *point* at other nodes - **Undirected** - where nodes are connected, but without direction (or bi-directionally, if you prefer) - **Degree** - the number of edges on a node - **Indegree** of a node - the number of arrows pointing to the node - **Outdegree** of a node - the number of arrows pointing out of the node - **Cycles** - loops - **Cyclic** - a graph that contains a loop you could go on foreer - **Acyclic** - a graph that contains no loops | Real term | Indegree | Outdegree | AKA terminology | | --------------- | -------- | --------- | ------------------------------------------- | | Isolated vertex | 0 | 0 | Island, disconnected node, Steven Glansberg | | Source | 0 | 1+ | Origin, root | | Sink | 1+ | 0 | Dead end, leaves | ## Graph Taxonomy | Number of Sources | Maximum parents per node | Terms | | ----------------- | ------------------------ | ---------------------------------------------- | | 1 | 1 | Tree, hierarchy, pure hierarchy, monohierarchy | | >1 | 1 | Forest, set of monohierarchies | | 1 | >1 | Single-rooted polyhierarchy, knotted tree | | >1 | >1 | Multi-rooted polyhierarchy, merging trees | All of those are also [[DAG|Directed Acyclic Graph]]s. ![[DAG 2026-09-13 11.00.58.excalidraw.svg]] %%[[DAG 2026-09-13 11.00.58.excalidraw.md|🖋 Edit in Excalidraw]]%% **** # More ## Source - Working with Google & ChatGPT [^1]: pun intended?