Set theory

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Across
  1. 3. A=B
  2. 4. {}
  3. 5. ℵ₁
  4. 7. ℤ
  5. 9. A∩B
  6. 12. objects that belong to A and not to B
  7. 14. ⊆
  8. 15. |A|
Down
  1. 1. ∅
  2. 2. ⊃
  3. 6. P(A)
  4. 8. all objects that do not belong to set A
  5. 10. a∈A
  6. 11. set of all ordered pairs from A to B
  7. 13. collection of two elements