Some relations among invariants of graphs
A remark on a problem of Harary
A De Bruijn-Erdős theorem for - metric spaces
A special case of a combinatorial theorem of De Bruijn and Erdős asserts that every noncollinear set of points in the plane determines at least distinct lines. Chen and Chvátal suggested a possible generalization of this assertion in metric spaces with appropriately defined lines. We prove this generalization in all metric spaces where each nonzero distance equals or .
Remark on a paper of Lovász
On finite and countable rigid graphs and tournaments
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