21865991 has 4 divisors (see below), whose sum is σ = 24989712. Its totient is φ = 18742272.

The previous prime is 21865981. The next prime is 21865993. The reversal of 21865991 is 19956812.

It is a happy number.

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 21865991 - 2¹⁸ = 21603847 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 (21865993) 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, 1561850 + ... + 1561863.

It is an arithmetic number, because the mean of its divisors is an integer number (6247428).

Almost surely, 2^21865991 is an apocalyptic number.

21865991 is a deficient number, since it is larger than the sum of its proper divisors (3123721).

21865991 is an equidigital number, since it uses as much as digits as its factorization.

21865991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3123720.

The product of its digits is 38880, while the sum is 41.

The square root of 21865991 is about 4676.1085316746. The cubic root of 21865991 is about 279.6338368440.

The spelling of 21865991 in words is "twenty-one million, eight hundred sixty-five thousand, nine hundred ninety-one".