99999999 is nontrivially palindromic in base 10.
It is a trimorphic number since its cube, 999999970000000299999999, ends in 99999999.
It is a nude number because it is divisible by every one of its digits.
99999999 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 (9999999800000001) can be partitioned into two parts whose sum is 99999999.
Almost surely, 299999999 is an apocalyptic number.
99999999 is a gapful number since it is divisible by the number (99) formed by its first and last digit.
99999999 is a wasteful number, since it uses less digits than its factorization.
99999999 is an odious number, because the sum of its binary digits is odd.
The square root of 99999999 is about 9999.9999500000. Note that the first 4 decimals coincide. The cubic root of 99999999 is about 464.1588818141.
The spelling of 99999999 in words is "ninety-nine million, nine hundred ninety-nine thousand, nine hundred ninety-nine".