21636721 has 4 divisors (see below), whose sum is σ = 22577472. Its totient is φ = 20695972.

The previous prime is 21636709. The next prime is 21636743. The reversal of 21636721 is 12763612.

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 21636721 - 2¹⁹ = 21112433 is a prime.

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

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 21636721.

It is not an unprimeable number, because it can be changed into a prime (21636781) 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 3 ways as a sum of consecutive naturals, for example, 470341 + ... + 470386.

It is an arithmetic number, because the mean of its divisors is an integer number (5644368).

Almost surely, 2^21636721 is an apocalyptic number.

It is an amenable number.

21636721 is a deficient number, since it is larger than the sum of its proper divisors (940751).

21636721 is an equidigital number, since it uses as much as digits as its factorization.

21636721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 940750.

The product of its digits is 3024, while the sum is 28.

The square root of 21636721 is about 4651.5288884409. The cubic root of 21636721 is about 278.6530589864.

The spelling of 21636721 in words is "twenty-one million, six hundred thirty-six thousand, seven hundred twenty-one".