21640321 has 4 divisors (see below), whose sum is σ = 21688132. Its totient is φ = 21592512.

The previous prime is 21640279. The next prime is 21640331. The reversal of 21640321 is 12304612.

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 9641025 + 11999296 = 3105^2 + 3464^2 .

It is a cyclic number.

It is not a de Polignac number, because 21640321 - 2⁹ = 21639809 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21640331) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 23220 + ... + 24133.

It is an arithmetic number, because the mean of its divisors is an integer number (5422033).

Almost surely, 2^21640321 is an apocalyptic number.

It is an amenable number.

21640321 is a deficient number, since it is larger than the sum of its proper divisors (47811).

21640321 is an equidigital number, since it uses as much as digits as its factorization.

21640321 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 47810.

The product of its (nonzero) digits is 288, while the sum is 19.

The square root of 21640321 is about 4651.9158418871. The cubic root of 21640321 is about 278.6685125816.

Adding to 21640321 its reverse (12304612), we get a palindrome (33944933).

The spelling of 21640321 in words is "twenty-one million, six hundred forty thousand, three hundred twenty-one".