21656521 has 8 divisors (see below), whose sum is σ = 23159304. Its totient is φ = 20179200.

The previous prime is 21656513. The next prime is 21656539. The reversal of 21656521 is 12565612.

It can be written as a sum of positive squares in 4 ways, for example, as 11022400 + 10634121 = 3320^2 + 3261^2 .

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

It is a cyclic number.

It is not a de Polignac number, because 21656521 - 2³ = 21656513 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is a self number, because there is not a number n which added to its sum of digits gives 21656521.

It is not an unprimeable number, because it can be changed into a prime (21656021) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 4590 + ... + 8023.

It is an arithmetic number, because the mean of its divisors is an integer number (2894913).

Almost surely, 2^21656521 is an apocalyptic number.

It is an amenable number.

21656521 is a deficient number, since it is larger than the sum of its proper divisors (1502783).

21656521 is a wasteful number, since it uses less digits than its factorization.

21656521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 12731.

The product of its digits is 3600, while the sum is 28.

The square root of 21656521 is about 4653.6567342252. The cubic root of 21656521 is about 278.7380325588.

Subtracting from 21656521 its reverse (12565612), we obtain a palindrome (9090909).

The spelling of 21656521 in words is "twenty-one million, six hundred fifty-six thousand, five hundred twenty-one".