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Bulgaria counting and probability
Problem
A graph is called divisibility graph if the vertices can be assigned distinct positive integers such that between two vertices assigned there is an edge iff or is a positive integer. Show that for any positive integer and , there is a divisibility graph with vertices and edges. (Danila Cherkashin)
Solution
We reason inductively on , not writing the number at any vertex. For the requested is clear, for an example with is and an example with is . For example with is , example with is , example with is , an example with is . For we have , so at least one of and is satisfied. Let first. In an example with vertices and edges, we add a vertex (of degree ) by writing in it a prime number greater than the numbers in the other vertices, then we multiply the numbers in the remaining vertices by – this does not spawn new edges between the remaining vertices. Now let . In an example with vertices and edges, we add a vertex (of degree ), writing in it a prime number greater than the numbers in the other vertices, and do not change the numbers in the other vertices – it does not spawn new edges.
Techniques
Graph TheoryPrime numbersInduction / smoothing