21778551 has 20 divisors (see below), whose sum is σ = 33027192. Its totient is φ = 14298768.

The previous prime is 21778543. The next prime is 21778573. The reversal of 21778551 is 15587712.

It is not a de Polignac number, because 21778551 - 2³ = 21778543 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21778501) by changing a digit.

It is a polite number, since it can be written in 19 ways as a sum of consecutive naturals, for example, 3421 + ... + 7433.

Almost surely, 2^21778551 is an apocalyptic number.

21778551 is a deficient number, since it is larger than the sum of its proper divisors (11248641).

21778551 is an equidigital number, since it uses as much as digits as its factorization.

21778551 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4092 (or 4083 counting only the distinct ones).

The product of its digits is 19600, while the sum is 36.

The square root of 21778551 is about 4666.7495111694. The cubic root of 21778551 is about 279.2605959218.

The spelling of 21778551 in words is "twenty-one million, seven hundred seventy-eight thousand, five hundred fifty-one".