Basic math symbols

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Across
  1. 4. 4√a
  2. 5. π
  3. 6. √a
  4. 7. ∑
  5. 11. a^b
  6. 12. %
  7. 13. ≠
  8. 14. mod
  9. 16. *
  10. 18. ⌈x⌉
  11. 19. set
  12. 20. <
Down
  1. 1. calculate expression inside first
  2. 2. exponent
  3. 3. ≈
  4. 8. φ
  5. 9. both plus and minus operations
  6. 10. absolute value
  7. 15. e = 2.718281828...
  8. 17. ≤