21511521 has 16 divisors (see below), whose sum is σ = 31942720. Its totient is φ = 14307840.

The previous prime is 21511519. The next prime is 21511531. The reversal of 21511521 is 12511512.

It is not a de Polignac number, because 21511521 - 2¹ = 21511519 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21511494 and 21511503.

It is not an unprimeable number, because it can be changed into a prime (21511531) 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 15 ways as a sum of consecutive naturals, for example, 18081 + ... + 19233.

It is an arithmetic number, because the mean of its divisors is an integer number (1996420).

Almost surely, 2^21511521 is an apocalyptic number.

It is an amenable number.

21511521 is a deficient number, since it is larger than the sum of its proper divisors (10431199).

21511521 is a wasteful number, since it uses less digits than its factorization.

21511521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1853 (or 1847 counting only the distinct ones).

The product of its digits is 100, while the sum is 18.

The square root of 21511521 is about 4638.0514227421. The cubic root of 21511521 is about 278.1145474505.

Subtracting from 21511521 its reverse (12511512), we obtain a palindrome (9000009).

The spelling of 21511521 in words is "twenty-one million, five hundred eleven thousand, five hundred twenty-one".