21809855 has 4 divisors (see below), whose sum is σ = 26171832. Its totient is φ = 17447880.

The previous prime is 21809789. The next prime is 21809863. The reversal of 21809855 is 55890812.

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

It is a de Polignac number, because none of the positive numbers 2^k-21809855 is a prime.

It is a Duffinian number.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2180981 + ... + 2180990.

It is an arithmetic number, because the mean of its divisors is an integer number (6542958).

Almost surely, 2^21809855 is an apocalyptic number.

21809855 is a deficient number, since it is larger than the sum of its proper divisors (4361977).

21809855 is an equidigital number, since it uses as much as digits as its factorization.

21809855 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4361976.

The product of its (nonzero) digits is 28800, while the sum is 38.

The square root of 21809855 is about 4670.1022472747. The cubic root of 21809855 is about 279.3943328346.

The spelling of 21809855 in words is "twenty-one million, eight hundred nine thousand, eight hundred fifty-five".