21991724 has 6 divisors (see below), whose sum is σ = 38485524. Its totient is φ = 10995860.

The previous prime is 21991699. The next prime is 21991733. The reversal of 21991724 is 42719912.

It is an unprimeable number.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 2748962 + ... + 2748969.

It is an arithmetic number, because the mean of its divisors is an integer number (6414254).

Almost surely, 2^21991724 is an apocalyptic number.

It is an amenable number.

21991724 is a deficient number, since it is larger than the sum of its proper divisors (16493800).

21991724 is a wasteful number, since it uses less digits than its factorization.

21991724 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5497935 (or 5497933 counting only the distinct ones).

The product of its digits is 9072, while the sum is 35.

The square root of 21991724 is about 4689.5334522743. The cubic root of 21991724 is about 280.1687927838.

The spelling of 21991724 in words is "twenty-one million, nine hundred ninety-one thousand, seven hundred twenty-four".