21987709 has 4 divisors (see below), whose sum is σ = 22098400. Its totient is φ = 21877020.

The previous prime is 21987703. The next prime is 21987727. The reversal of 21987709 is 90778912.

It is a happy number.

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.

It is a cyclic number.

It is not a de Polignac number, because 21987709 - 2⁹ = 21987197 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21987703) 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, 55047 + ... + 55444.

It is an arithmetic number, because the mean of its divisors is an integer number (5524600).

Almost surely, 2^21987709 is an apocalyptic number.

It is an amenable number.

21987709 is a deficient number, since it is larger than the sum of its proper divisors (110691).

21987709 is a wasteful number, since it uses less digits than its factorization.

21987709 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 110690.

The product of its (nonzero) digits is 63504, while the sum is 43.

The square root of 21987709 is about 4689.1053517702. The cubic root of 21987709 is about 280.1517417306.

The spelling of 21987709 in words is "twenty-one million, nine hundred eighty-seven thousand, seven hundred nine".