21657987 has 24 divisors (see below), whose sum is σ = 34607664. Its totient is φ = 12965184.

The previous prime is 21657971. The next prime is 21657991. The reversal of 21657987 is 78975612.

It is not a de Polignac number, because 21657987 - 2⁴ = 21657971 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21657487) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 1828 + ... + 6830.

It is an arithmetic number, because the mean of its divisors is an integer number (1441986).

Almost surely, 2^21657987 is an apocalyptic number.

21657987 is a deficient number, since it is larger than the sum of its proper divisors (12949677).

21657987 is a wasteful number, since it uses less digits than its factorization.

21657987 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5059 (or 5056 counting only the distinct ones).

The product of its digits is 211680, while the sum is 45.

The square root of 21657987 is about 4653.8142421029. The cubic root of 21657987 is about 278.7443219767.

The spelling of 21657987 in words is "twenty-one million, six hundred fifty-seven thousand, nine hundred eighty-seven".