It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 1279999 = 7 ⋅182857.
It is a cyclic number.
It is an Ulam number.
It is a Duffinian number.
9999721 is a lucky number.
It is a nialpdrome in base 10.
Almost surely, 29999721 is an apocalyptic number.
It is an amenable number.
9999721 is a wasteful number, since it uses less digits than its factorization.
9999721 is an odious number, because the sum of its binary digits is odd.
The sum of its prime factors is 6550.
The square root of 9999721 is about 3162.2335460873. The cubic root of 9999721 is about 215.4414653603.
The spelling of 9999721 in words is "nine million, nine hundred ninety-nine thousand, seven hundred twenty-one".