21023521 has 4 divisors (see below), whose sum is σ = 21748500. Its totient is φ = 20298544.

The previous prime is 21023503. The next prime is 21023531. The reversal of 21023521 is 12532012.

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 11189025 + 9834496 = 3345^2 + 3136^2 .

It is a cyclic number.

It is not a de Polignac number, because 21023521 - 2⁷ = 21023393 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21023495 and 21023504.

It is not an unprimeable number, because it can be changed into a prime (21023501) 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, 362446 + ... + 362503.

It is an arithmetic number, because the mean of its divisors is an integer number (5437125).

Almost surely, 2^21023521 is an apocalyptic number.

It is an amenable number.

21023521 is a deficient number, since it is larger than the sum of its proper divisors (724979).

21023521 is an equidigital number, since it uses as much as digits as its factorization.

21023521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 724978.

The product of its (nonzero) digits is 120, while the sum is 16.

The square root of 21023521 is about 4585.1413282471. The cubic root of 21023521 is about 275.9953834206.

Adding to 21023521 its reverse (12532012), we get a palindrome (33555533).

The spelling of 21023521 in words is "twenty-one million, twenty-three thousand, five hundred twenty-one".