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Summary
DescriptionCirculation in connected regions - schematic.png
English: Left: The curve C traces out a closed path around a point P in a simply connected region. The path C can be shrunken down down to the point C, allowing the use of Stokes theorem.
Right: The curve C traces a closed path in a multiply-connected region (i.e it contains holes). The path C cannot be shrunk down to the point P (at the centre of the circle) and therefore cannot apply Stokes theorem.
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Captions
The circulation in a simply-connected region and a multiply connected region.