21006521 has 4 divisors (see below), whose sum is σ = 21919872. Its totient is φ = 20093172.

The previous prime is 21006511. The next prime is 21006523. The reversal of 21006521 is 12560012.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21006521 - 2¹⁸ = 20744377 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21006521.

It is not an unprimeable number, because it can be changed into a prime (21006523) 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, 456641 + ... + 456686.

It is an arithmetic number, because the mean of its divisors is an integer number (5479968).

Almost surely, 2^21006521 is an apocalyptic number.

It is an amenable number.

21006521 is a deficient number, since it is larger than the sum of its proper divisors (913351).

21006521 is an equidigital number, since it uses as much as digits as its factorization.

21006521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 913350.

The product of its (nonzero) digits is 120, while the sum is 17.

The square root of 21006521 is about 4583.2871391612. The cubic root of 21006521 is about 275.9209717376.

Adding to 21006521 its reverse (12560012), we get a palindrome (33566533).

The spelling of 21006521 in words is "twenty-one million, six thousand, five hundred twenty-one".