21056721 has 12 divisors (see below), whose sum is σ = 32659632. Its totient is φ = 12032328.

The previous prime is 21056701. The next prime is 21056723. The reversal of 21056721 is 12765012.

It is not a de Polignac number, because 21056721 - 2⁵ = 21056689 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is a junction number, because it is equal to n+sod(n) for n = 21056691 and 21056700.

It is not an unprimeable number, because it can be changed into a prime (21056723) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 71475 + ... + 71768.

It is an arithmetic number, because the mean of its divisors is an integer number (2721636).

Almost surely, 2^21056721 is an apocalyptic number.

21056721 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21056721 is a deficient number, since it is larger than the sum of its proper divisors (11602911).

21056721 is a wasteful number, since it uses less digits than its factorization.

21056721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 143260 (or 143253 counting only the distinct ones).

The product of its (nonzero) digits is 840, while the sum is 24.

The square root of 21056721 is about 4588.7602901001. The cubic root of 21056721 is about 276.1405894747.

The spelling of 21056721 in words is "twenty-one million, fifty-six thousand, seven hundred twenty-one".