Degree-diameter problem $N(13,4)$
The largest possible number of vertices in a graph of maximum degree $13$ and diameter $4$.
Lower bound:
$6560$
Upper bound:
Unknown
Updates
-
1985
Lower bound: $6560$
Gómez, José; Fiol, Miquel (1985), Dense compound graphs, Ars Combinatoria, 20: 211-237
[via Table of the largest known graphs of a given diameter and maximal degree - Wikipedia]