= Seven Bridges of Königsberg
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Model the four land regions by <vertices> $A,B,C,D$ and the seven bridges by distinct <edges>: two $AB$, two $AC$, and one each of $AD,BD,CD$. Their <vertex degrees> are $(5,3,3,3)$. <Leonhard Euler>'s paper records these connections in §6 and the obstruction in §9; §20 states the zero-or-two odd-region rule. Four odd <vertex degrees> exclude an <Eulerian trail>, even if returning to the starting <vertex> is optional.<Solutio problematis ad geometriam situs pertinentis by Leonhard Euler>[§§6, 9, 20]{ref}
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