Daily tweets about networks, graph theory, and related topics from @JohnDCook

Joined July 2013
In many real-world networks, average distances are on the order of the log of the number of vertices.
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1,052
Two nodes x and y in a directed graph are strongly connected if there is a path from x to y and a path from y to x.
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1,353
Two nodes x and y in a directed graph are weakly connected if there is a path from x to y or from y to x.
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1,345
Gallery of named graphs altermundus.fr/downloads/doc… // 104-page pdf from Alain Matthes
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1,464
A connected graph contains an Eulerian path iff there are zero or two nodes of odd degree.
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1,595
A Hamiltonian path visits each vertex in a graph exactly once.
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1,470
An Eulerian path traverses each edge in a graph exactly once.
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Some authors say graph theory started with Euler's solution to the Kӧnigsberg bridge problem in 1736.
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1,478
The diameter of a connected graph is the length of the longest geodesic path in the graph.
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1,630
If A is the adjacency matrix of a graph, the number of walks from i to j of length r is the ij-th entry of A^r.
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1,501
The density of a graph is the ratio of the actual number of edges to the maximum possible number of edges.
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1,495
The degree of a vertex is the number of edges connected to it.
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1,534
The number of spanning trees in a connected network w/ n nodes equals the product of the nonzero eigenvalues of its Laplacian, divided by n.
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1,550