What is the hardest math problem?

Related questions

1. Is Linear Algebra harder than algebra?
2. How useful is linear algebra?
3. Is Linear Algebra easy?
4. Does linear algebra come after calculus?
5. What is linear algebra and matrix theory?
6. Is discrete math like linear algebra?
7. Is Linear Algebra algebra?
8. Is discrete math useful?
9. Is matrix algebra the same as linear algebra?
10. Why is calculus taught before linear algebra?

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