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Jordan Curve Theorem: If you have a closed curve, there is an "inside" and "outside", and those are defined globally (two points are on different sides if every path between them crosses the curve at least once), but one can test whether two points are on the same side with a somewhat local test of drawing a line between them. If the line intersects the ...
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Local-to-global is a big thing in algebra, starting with (Hasse-)Minkowski's result that if you have a solution to a quadratic form modulo all prime powers and in the reals, you also have one over the integers. (See Theorem 2.4 here for the Minkowskian statement without $p$-adic numbers, which is otherwise a bit difficult to find in a quick web search.)
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One idea that comes to mind is the fact that a connected graph has a Eulerian circuit if and only if every vertex has even degree.
Another idea that comes to mind is just the Fundamental Theorem of Calculus. If you have a rather boring video of a car's speedometer as the car travels from A to B, then you can figure out the total distance the car has ...
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Here are two similar ideas.
(1) The shortest geodesic between two points on the surface of a polyhedron (generally) bends as it crosses edges, when viewed
globally in $\mathbb{R}^3$, but from the local point of view of an ant walking along the path, it is straight, i.e., straight when each crossed edge is unfolded flat.
This is commonly illustrated on a ...
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