21604221 has 6 divisors (see below), whose sum is σ = 31206110. Its totient is φ = 14402808.

The previous prime is 21604213. The next prime is 21604223. The reversal of 21604221 is 12240612.

It can be written as a sum of positive squares in only one way, i.e., 19263321 + 2340900 = 4389^2 + 1530^2 .

It is not a de Polignac number, because 21604221 - 2³ = 21604213 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a junction number, because it is equal to n+sod(n) for n = 21604194 and 21604203.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21604223) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 1200226 + ... + 1200243.

Almost surely, 2^21604221 is an apocalyptic number.

It is an amenable number.

21604221 is a deficient number, since it is larger than the sum of its proper divisors (9601889).

21604221 is a wasteful number, since it uses less digits than its factorization.

21604221 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2400475 (or 2400472 counting only the distinct ones).

The product of its (nonzero) digits is 192, while the sum is 18.

The square root of 21604221 is about 4648.0341005634. The cubic root of 21604221 is about 278.5134697116.

Adding to 21604221 its reverse (12240612), we get a palindrome (33844833).

The spelling of 21604221 in words is "twenty-one million, six hundred four thousand, two hundred twenty-one".