Graphs

Graphs are sets of nodes and edges.  They are useful to study pair sets.  Graphs can have multiple edges between the same node pairs.  Edges can have weights, directions and form loops.  Cycles are graph paths, with no repeated edges, that arrive back at their starting nodes.  Acyclic graphs do not have loops and cycles.  Trees are acyclic graphs.

Transformers, sentiment analyzers, search engines and recommendation systems use graphs.