21551999 has 4 divisors (see below), whose sum is σ = 24630864. Its totient is φ = 18473136.

The previous prime is 21551993. The next prime is 21552007. The reversal of 21551999 is 99915512.

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 21551999 - 2⁴ = 21551983 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 (21551993) 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, 1539422 + ... + 1539435.

It is an arithmetic number, because the mean of its divisors is an integer number (6157716).

Almost surely, 2^21551999 is an apocalyptic number.

21551999 is a deficient number, since it is larger than the sum of its proper divisors (3078865).

21551999 is an equidigital number, since it uses as much as digits as its factorization.

21551999 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3078864.

The product of its digits is 36450, while the sum is 41.

The square root of 21551999 is about 4642.4130578827. The cubic root of 21551999 is about 278.2888798771.

The spelling of 21551999 in words is "twenty-one million, five hundred fifty-one thousand, nine hundred ninety-nine".