999999999 is nontrivially palindromic in base 10.
It is a trimorphic number since its cube, 999999997000000002999999999, ends in 999999999.
It is a nude number because it is divisible by every one of its digits.
It is a Duffinian number.
999999999 is a nontrivial repdigit in base 10.
It is a plaindrome in base 10.
It is a nialpdrome in base 10.
It is a zygodrome in base 10.
It is a congruent number.
It is a Kaprekar number, because its square (999999998000000001) can be partitioned into two parts whose sum is 999999999.
Almost surely, 2999999999 is an apocalyptic number.
999999999 is a wasteful number, since it uses less digits than its factorization.
999999999 is an odious number, because the sum of its binary digits is odd.
The square root of 999999999 is about 31622.7765858724. The cubic root of 999999999 is about 999.9999996667. Note that the first 6 decimals are identical.
The spelling of 999999999 in words is "nine hundred ninety-nine million, nine hundred ninety-nine thousand, nine hundred ninety-nine".