Safety in Numbers

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Across
  1. 3. sweet mathematicians dream about
  2. 6. same on both sides
  3. 7. touch at only one point
  4. 9. meet only when right and not when wrong
  5. 11. a+b=b+a
  6. 15. rate of change
  7. 17. two circles and a flat curved surface
  8. 18. never meet
  9. 19. number six was afraid of
Down
  1. 1. double, double, and keep doubling until we run out of numbers
  2. 2. four equal sides that meet all wrong
  3. 4. part of the quadratic formula
  4. 5. special relation where each input value has at most one output value
  5. 8. climbed uphill then rollerbladed downhill
  6. 10. numbers that never terminate nor have a pattern
  7. 12. can't touch this, well usually not
  8. 13. numbers that are only divisible by one and itself
  9. 14. complimenti sinus
  10. 16. number system we imagine exists
  11. 17. pointy and great for ice cream