21604421 has 2 divisors, whose sum is σ = 21604422. Its totient is φ = 21604420.

The previous prime is 21604397. The next prime is 21604439. The reversal of 21604421 is 12440612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21520321 + 84100 = 4639^2 + 290^2 .

It is a cyclic number.

It is not a de Polignac number, because 21604421 - 2¹⁰ = 21603397 is a prime.

It is a Sophie Germain prime.

It is a Chen prime.

It is a Curzon number.

It is a junction number, because it is equal to n+sod(n) for n = 21604393 and 21604402.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21604481) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10802210 + 10802211.

It is an arithmetic number, because the mean of its divisors is an integer number (10802211).

Almost surely, 2^21604421 is an apocalyptic number.

It is an amenable number.

21604421 is a deficient number, since it is larger than the sum of its proper divisors (1).

21604421 is an equidigital number, since it uses as much as digits as its factorization.

21604421 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 384, while the sum is 20.

The square root of 21604421 is about 4648.0556149857. The cubic root of 21604421 is about 278.5143291504.

The spelling of 21604421 in words is "twenty-one million, six hundred four thousand, four hundred twenty-one".