21645721 has 4 divisors (see below), whose sum is σ = 21678372. Its totient is φ = 21613072.

The previous prime is 21645707. The next prime is 21645733. The reversal of 21645721 is 12754612.

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 21261321 + 384400 = 4611^2 + 620^2 .

It is a cyclic number.

It is not a de Polignac number, because 21645721 - 2¹⁵ = 21612953 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21645721.

It is not an unprimeable number, because it can be changed into a prime (21645751) 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, 15310 + ... + 16663.

It is an arithmetic number, because the mean of its divisors is an integer number (5419593).

Almost surely, 2^21645721 is an apocalyptic number.

It is an amenable number.

21645721 is a deficient number, since it is larger than the sum of its proper divisors (32651).

21645721 is an equidigital number, since it uses as much as digits as its factorization.

21645721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 32650.

The product of its digits is 3360, while the sum is 28.

The square root of 21645721 is about 4652.4962117126. The cubic root of 21645721 is about 278.6916897612.

It can be divided in two parts, 21645 and 721, that added together give a triangular number (22366 = T₂₁₁).

The spelling of 21645721 in words is "twenty-one million, six hundred forty-five thousand, seven hundred twenty-one".