21056821 has 8 divisors (see below), whose sum is σ = 21634200. Its totient is φ = 20482560.

The previous prime is 21056801. The next prime is 21056831. The reversal of 21056821 is 12865012.

It can be written as a sum of positive squares in 4 ways, for example, as 680625 + 20376196 = 825^2 + 4514^2 .

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

It is a cyclic number.

It is not a de Polignac number, because 21056821 - 2¹¹ = 21054773 is a prime.

It is a super-3 number, since 3×21056821³ (a number of 23 digits) contains 333 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21056801) 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, 20365 + ... + 21373.

It is an arithmetic number, because the mean of its divisors is an integer number (2704275).

Almost surely, 2^21056821 is an apocalyptic number.

It is an amenable number.

21056821 is a deficient number, since it is larger than the sum of its proper divisors (577379).

21056821 is a wasteful number, since it uses less digits than its factorization.

21056821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1559.

The product of its (nonzero) digits is 960, while the sum is 25.

The square root of 21056821 is about 4588.7711862763. The cubic root of 21056821 is about 276.1410266117.

The spelling of 21056821 in words is "twenty-one million, fifty-six thousand, eight hundred twenty-one".