21605521 has 24 divisors (see below), whose sum is σ = 27369576. Its totient is φ = 16934400.

The previous prime is 21605509. The next prime is 21605531. The reversal of 21605521 is 12550612.

It can be written as a sum of positive squares in 4 ways, for example, as 14714896 + 6890625 = 3836^2 + 2625^2 .

It is not a de Polignac number, because 21605521 - 2⁹ = 21605009 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21605492 and 21605501.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21605501) 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 23 ways as a sum of consecutive naturals, for example, 30471 + ... + 31171.

It is an arithmetic number, because the mean of its divisors is an integer number (1140399).

Almost surely, 2^21605521 is an apocalyptic number.

It is an amenable number.

21605521 is a deficient number, since it is larger than the sum of its proper divisors (5764055).

21605521 is a wasteful number, since it uses less digits than its factorization.

21605521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 769 (or 762 counting only the distinct ones).

The product of its (nonzero) digits is 600, while the sum is 22.

The square root of 21605521 is about 4648.1739425284. The cubic root of 21605521 is about 278.5190559694.

The spelling of 21605521 in words is "twenty-one million, six hundred five thousand, five hundred twenty-one".