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The Graham-Pollak Theorem states that the minimum number of complete bipartite graphs needed to disjointly cover the edge set of the complete graph on n vertices is n-1. The only known proofs all use linear algebra, and there os is no pure counting proof is known as far as I know. There is a chapter about this in Aigner and Ziegler's Proofs from the BOOK.

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The Graham-Pollak Theorem states that the minimum number of complete bipartite graphs needed to disjointly cover the edge set of the complete graph on n vertices is n-1. The only known proofs all use linear algebra, and there os no pure counting proof is known as far as I know. There is a chapter about this in Aigner and Ziegler's Proofs from the BOOK.