21815521 has 8 divisors (see below), whose sum is σ = 26849984. Its totient is φ = 17260560.

The previous prime is 21815483. The next prime is 21815527. The reversal of 21815521 is 12551812.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21815521 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21815527) 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 7 ways as a sum of consecutive naturals, for example, 119775 + ... + 119956.

It is an arithmetic number, because the mean of its divisors is an integer number (3356248).

Almost surely, 2^21815521 is an apocalyptic number.

It is an amenable number.

21815521 is a deficient number, since it is larger than the sum of its proper divisors (5034463).

21815521 is a wasteful number, since it uses less digits than its factorization.

21815521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 239751.

The product of its digits is 800, while the sum is 25.

The square root of 21815521 is about 4670.7088327148. The cubic root of 21815521 is about 279.4185254335.

The spelling of 21815521 in words is "twenty-one million, eight hundred fifteen thousand, five hundred twenty-one".