What is the hardest branch of mathematics?

Related questions

1. Is calculus difficult to learn?
2. Do you need calculus for linear algebra?
3. Is linear algebra used in statistics?
4. Is discrete math like linear algebra?
5. What is the difference between linear algebra and calculus?
6. Is calculus easier than algebra?
7. Is Linear Algebra harder than algebra?
8. Is trigonometry easier than algebra?
9. Why do we need linear algebra?
10. Is Linear Algebra easy?

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2. #analysis
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18. #originally