21680221 has 4 divisors (see below), whose sum is σ = 21744892. Its totient is φ = 21615552.

The previous prime is 21680203. The next prime is 21680227. The reversal of 21680221 is 12208612.

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 15100996 + 6579225 = 3886^2 + 2565^2 .

It is a cyclic number.

It is not a de Polignac number, because 21680221 - 2⁵ = 21680189 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21680192 and 21680201.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21680227) 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, 31830 + ... + 32503.

It is an arithmetic number, because the mean of its divisors is an integer number (5436223).

Almost surely, 2^21680221 is an apocalyptic number.

It is an amenable number.

21680221 is a deficient number, since it is larger than the sum of its proper divisors (64671).

21680221 is an equidigital number, since it uses as much as digits as its factorization.

21680221 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 64670.

The product of its (nonzero) digits is 384, while the sum is 22.

The square root of 21680221 is about 4656.2024225757. The cubic root of 21680221 is about 278.8396752776.

Adding to 21680221 its reverse (12208612), we get a palindrome (33888833).

The spelling of 21680221 in words is "twenty-one million, six hundred eighty thousand, two hundred twenty-one".