It is an Ulam number.
It is a Curzon number.
It is a self number, because there is not a number n which added to its sum of digits gives 999978.
It is a congruent number.
It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 999978.
2999978 is an apocalyptic number.
It is a practical number, because each smaller number is the sum of distinct divisors of 999978, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (1183680).
It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.
999978 is a wasteful number, since it uses less digits than its factorization.
999978 is an odious number, because the sum of its binary digits is odd.
The sum of its prime factors is 862.
The square root of 999978 is about 999.9889999395. The cubic root of 999978 is about 99.9992666613. Note that the first 3 decimals are identical.
The spelling of 999978 in words is "nine hundred ninety-nine thousand, nine hundred seventy-eight".