21997987 has 4 divisors (see below), whose sum is σ = 23997816. Its totient is φ = 19998160.

The previous prime is 21997981. The next prime is 21998027. The reversal of 21997987 is 78979912.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21997987 - 2¹⁵ = 21965219 is a prime.

It is a super-2 number, since 2×21997987² = 967822864104338, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21997981) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 999898 + ... + 999919.

It is an arithmetic number, because the mean of its divisors is an integer number (5999454).

Almost surely, 2^21997987 is an apocalyptic number.

21997987 is a deficient number, since it is larger than the sum of its proper divisors (1999829).

21997987 is a wasteful number, since it uses less digits than its factorization.

21997987 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1999828.

The product of its digits is 571536, while the sum is 52.

The square root of 21997987 is about 4690.2011683935. The cubic root of 21997987 is about 280.1953865849.

The spelling of 21997987 in words is "twenty-one million, nine hundred ninety-seven thousand, nine hundred eighty-seven".