999999 is nontrivially palindromic in base 10.
It is a trimorphic number since its cube, 999997000002999999, ends in 999999.
It is a nude number because it is divisible by every one of its digits.
999999 is a nontrivial repdigit in base 10.
It is a plaindrome in base 5 and 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 (999998000001) can be partitioned into two parts whose sum is 999999.
2999999 is an apocalyptic number.
999999 is a gapful number since it is divisible by the number (99) formed by its first and last digit.
999999 is a primitive abundant number, since it is smaller than the sum of its proper divisors, none of which is abundant.
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
999999 is a wasteful number, since it uses less digits than its factorization.
999999 is an evil number, because the sum of its binary digits is even.
The square root of 999999 is about 999.9994999999. Note that the first 3 decimals coincide. The cubic root of 999999 is about 99.9999666667. Note that the first 4 decimals are identical.
The spelling of 999999 in words is "nine hundred ninety-nine thousand, nine hundred ninety-nine".