21720555 has 56 divisors (see below), whose sum is σ = 40134960. Its totient is φ = 11275200.

The previous prime is 21720511. The next prime is 21720563. The reversal of 21720555 is 55502712.

21720555 = T₂ + T₃ + ... + T₅₀₆.

It is a happy number.

It is not a de Polignac number, because 21720555 - 2⁷ = 21720427 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is an unprimeable number.

It is a polite number, since it can be written in 55 ways as a sum of consecutive naturals, for example, 215005 + ... + 215105.

Almost surely, 2^21720555 is an apocalyptic number.

21720555 is a deficient number, since it is larger than the sum of its proper divisors (18414405).

21720555 is an equidigital number, since it uses as much as digits as its factorization.

21720555 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 183 (or 168 counting only the distinct ones).

The product of its (nonzero) digits is 3500, while the sum is 27.

The square root of 21720555 is about 4660.5316220363. The cubic root of 21720555 is about 279.0124864339.

The spelling of 21720555 in words is "twenty-one million, seven hundred twenty thousand, five hundred fifty-five".