What is the integral sign with a circle through it?

∮ = Contour Integral

http://math.fullerton.edu/mathews/c2003/ContourInt...

https://en.wikipedia.org/wiki/Methods_of_contour_i...

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thats called a path integral and its introduced in calculus 3 generally. its used within physical systems, things that go in a certain closed path, things like electricity. and if i remember correctly in the integral equates to 0 it means that the path is like really simple or something to that effect.