21805551 has 8 divisors (see below), whose sum is σ = 32304560. Its totient is φ = 14537016.

The previous prime is 21805549. The next prime is 21805561. The reversal of 21805551 is 15550812.

It is not a de Polignac number, because 21805551 - 2¹ = 21805549 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27), and also a Moran number because the ratio is a prime number: 807613 = 21805551 / (2 + 1 + 8 + 0 + 5 + 5 + 5 + 1).

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21805561) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 403780 + ... + 403833.

It is an arithmetic number, because the mean of its divisors is an integer number (4038070).

Almost surely, 2^21805551 is an apocalyptic number.

21805551 is a deficient number, since it is larger than the sum of its proper divisors (10499009).

21805551 is an equidigital number, since it uses as much as digits as its factorization.

21805551 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 807622 (or 807616 counting only the distinct ones).

The product of its (nonzero) digits is 2000, while the sum is 27.

The square root of 21805551 is about 4669.6414209230. The cubic root of 21805551 is about 279.3759528818.

The spelling of 21805551 in words is "twenty-one million, eight hundred five thousand, five hundred fifty-one".