21978021 has 16 divisors (see below), whose sum is σ = 31643136. Its totient is φ = 13488576.

The previous prime is 21978017. The next prime is 21978023. The reversal of 21978021 is 12087912.

It is not a de Polignac number, because 21978021 - 2² = 21978017 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21978021.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21978023) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 25036 + ... + 25898.

It is an arithmetic number, because the mean of its divisors is an integer number (1977696).

Almost surely, 2^21978021 is an apocalyptic number.

It is an amenable number.

21978021 is a deficient number, since it is larger than the sum of its proper divisors (9665115).

21978021 is a wasteful number, since it uses less digits than its factorization.

21978021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1532.

The product of its (nonzero) digits is 2016, while the sum is 30.

The square root of 21978021 is about 4688.0722050753. The cubic root of 21978021 is about 280.1105898187.

The spelling of 21978021 in words is "twenty-one million, nine hundred seventy-eight thousand, twenty-one".