mathlib

6b556cf1 - feat(combinatorics/adjacency_matrix): defines adjacency matrices of simple graphs (#3672)

feat(combinatorics/adjacency_matrix): defines adjacency matrices of simple graphs (#3672) defines the adjacency matrix of a simple graph proves lemmas about adjacency matrix that will form the bulk of the proof of the friendship theorem (freek 83) Co-authored-by: Aaron Anderson <awainverse@gmail.com>