Seven

Six fears it, nine is it's food: seven.

Properties

Seven is a mersenne prime: (2^3)-1 = 7. It's perfect number is 28 = (2^2)(7). It's a safe prime, as (2^((2^2)-1)-1 = 7.
7 is the sum of 8's proper divisors: 1+2+4.
7! = 7*6*5*4*3*2*1 = 5040, the last superior highly composite factorial.

For different bases

  1. In binary, 7 is 111, a repunit!
  2. In ternary, 7 is 21.
  3. In quaternary, 7 is 13.
  4. In quinary, 7 is 12.
  5. In seximal, 7 is 11.
  6. In septimal, 7 is 10.
  7. In higher bases, 7.
  8. In negabinary, 7 is 11011
    and -7 is 1001.
  9. In negaternary, 7 is 111
    and -7 is 1202.
  10. In negaquaternary, 7 is 133
    and -7 is 21.
  11. In negaquinary, 7 is 143
    and -7 is 23.
  12. In negaseximal, 7 is 152
    and -7 is 25.
  13. In negaseptimal, 7 is 160
    and -7 is 10.

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