21052721 has 4 divisors (see below), whose sum is σ = 21566244. Its totient is φ = 20539200.

The previous prime is 21052711. The next prime is 21052729. The reversal of 21052721 is 12725012.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 2217121 + 18835600 = 1489^2 + 4340^2 .

It is a cyclic number.

It is not a de Polignac number, because 21052721 - 2¹⁸ = 20790577 is a prime.

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

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21052693 and 21052702.

It is not an unprimeable number, because it can be changed into a prime (21052729) 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 3 ways as a sum of consecutive naturals, for example, 256700 + ... + 256781.

It is an arithmetic number, because the mean of its divisors is an integer number (5391561).

Almost surely, 2^21052721 is an apocalyptic number.

It is an amenable number.

21052721 is a deficient number, since it is larger than the sum of its proper divisors (513523).

21052721 is an equidigital number, since it uses as much as digits as its factorization.

21052721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 513522.

The product of its (nonzero) digits is 280, while the sum is 20.

The square root of 21052721 is about 4588.3244218342. The cubic root of 21052721 is about 276.1231028599.

Adding to 21052721 its reverse (12725012), we get a palindrome (33777733).

The spelling of 21052721 in words is "twenty-one million, fifty-two thousand, seven hundred twenty-one".