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.