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Paths, cycles, trees, Euler and Hamilton
Connectivity, spanning trees, Euler circuits and the bridges of Königsberg.
A tree is a connected graph with no cycles: n vertices, n − 1 edges. Euler proved a graph has a circuit using every edge once exactly when every degree is even. The Königsberg bridges failed. Picture it: the seven bridges; four land masses with odd degree. Think it: Euler's condition is local (degrees) yet decides a global question, the first theorem of topology.
Работен пример · 5 choose 2
Стъпка по стъпка
- \binom{5}{2} = \frac{5!}{2!\,(5-2)!}
Unordered selections: n! / (k! (n−k)!).
- = \frac{120}{2 \times 6} = 10
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Как да се: Paths, cycles, trees, Euler and Hamilton
- Unordered selections: n! / (k! (n−k)!).
Въпроси, които хората задават
Permutation or combination?
Ask whether order matters. A lock code is a permutation (order matters); a hand of cards is a combination (it does not).
What is a graph in this sense?
Dots (vertices) joined by lines (edges), not a plot. Road maps, social networks and molecules are graphs; questions like "is there a route" and "how few colours" are graph theory.
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