21637721 has 8 divisors (see below), whose sum is σ = 25526784. Its totient is φ = 17948160.

The previous prime is 21637697. The next prime is 21637739. The reversal of 21637721 is 12773612.

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 21637721 - 2⁶ = 21637657 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21637921) 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, 49640 + ... + 50073.

It is an arithmetic number, because the mean of its divisors is an integer number (3190848).

Almost surely, 2^21637721 is an apocalyptic number.

It is an amenable number.

21637721 is a deficient number, since it is larger than the sum of its proper divisors (3889063).

21637721 is an equidigital number, since it uses as much as digits as its factorization.

21637721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 99751.

The product of its digits is 3528, while the sum is 29.

The square root of 21637721 is about 4651.6363787381. The cubic root of 21637721 is about 278.6573518237.

The spelling of 21637721 in words is "twenty-one million, six hundred thirty-seven thousand, seven hundred twenty-one".