It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 124999999 = 499 ⋅250501.
It is a cyclic number.
It is a Duffinian number.
It is a nialpdrome in base 10.
It is a self number, because there is not a number n which added to its sum of digits gives 999999421.
It is a congruent number.
Almost surely, 2999999421 is an apocalyptic number.
It is an amenable number.
999999421 is a wasteful number, since it uses less digits than its factorization.
999999421 is an evil number, because the sum of its binary digits is even.
The sum of its prime factors is 106910.
The square root of 999999421 is about 31622.7674468886. The cubic root of 999999421 is about 999.9998070000. Note that the first 3 decimals are identical.
The spelling of 999999421 in words is "nine hundred ninety-nine million, nine hundred ninety-nine thousand, four hundred twenty-one".