21055551 has 8 divisors (see below), whose sum is σ = 30626304. Its totient is φ = 12760920.

The previous prime is 21055543. The next prime is 21055561. The reversal of 21055551 is 15555012.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21055551 - 2³ = 21055543 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21055501) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 318991 + ... + 319056.

It is an arithmetic number, because the mean of its divisors is an integer number (3828288).

Almost surely, 2^21055551 is an apocalyptic number.

21055551 is a deficient number, since it is larger than the sum of its proper divisors (9570753).

21055551 is a wasteful number, since it uses less digits than its factorization.

21055551 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 638061.

The product of its (nonzero) digits is 1250, while the sum is 24.

The square root of 21055551 is about 4588.6328029164. The cubic root of 21055551 is about 276.1354748690.

The spelling of 21055551 in words is "twenty-one million, fifty-five thousand, five hundred fifty-one".