@no_TL In September, I was studying the "inverse geometry" https://en.wikipedia.org/wiki/Inversive_geometry of Japanese mathematics "Wasan" https://en.wikipedia.org/wiki/Japanese_mathematics. I thought the following image was the simplest and most beautiful result #現代和算
Choose a random graph with countably infinite vertices by flipping a coin to decide whether to include each edge. Or, construct a graph with binary numbers as vertices, with an edge \(x\)—\(y\) when \(x<y\) and the \(x\)th bit of \(y\) is one. Or, construct a graph on primes congruent to 1 mod 4, with an edge when one is a quadratic residue mod the other. They're all the same graph, the Rado graph (https://en.wikipedia.org/wiki/Rado_graph)! It has many other amazing properties. Now a Good Article on Wikipedia.
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