21561555 has 16 divisors (see below), whose sum is σ = 34575264. Its totient is φ = 11473920.

The previous prime is 21561503. The next prime is 21561571. The reversal of 21561555 is 55516512.

It is not a de Polignac number, because 21561555 - 2⁷ = 21561427 is a prime.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 6787 + ... + 9443.

It is an arithmetic number, because the mean of its divisors is an integer number (2160954).

Almost surely, 2^21561555 is an apocalyptic number.

21561555 is a deficient number, since it is larger than the sum of its proper divisors (13013709).

21561555 is a wasteful number, since it uses less digits than its factorization.

21561555 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3206.

The product of its digits is 7500, while the sum is 30.

The square root of 21561555 is about 4643.4421499573. The cubic root of 21561555 is about 278.3300042231.

The spelling of 21561555 in words is "twenty-one million, five hundred sixty-one thousand, five hundred fifty-five".