21638729 has 8 divisors (see below), whose sum is σ = 24758208. Its totient is φ = 18526320.

The previous prime is 21638723. The next prime is 21638759. The reversal of 21638729 is 92783612.

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 21638729 - 2²⁰ = 20590153 is a prime.

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

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21638692 and 21638701.

It is not an unprimeable number, because it can be changed into a prime (21638723) 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, 10394 + ... + 12300.

It is an arithmetic number, because the mean of its divisors is an integer number (3094776).

Almost surely, 2^21638729 is an apocalyptic number.

It is an amenable number.

21638729 is a deficient number, since it is larger than the sum of its proper divisors (3119479).

21638729 is a wasteful number, since it uses less digits than its factorization.

21638729 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3535.

The product of its digits is 36288, while the sum is 38.

The square root of 21638729 is about 4651.7447264440. The cubic root of 21638729 is about 278.6616788698.

The spelling of 21638729 in words is "twenty-one million, six hundred thirty-eight thousand, seven hundred twenty-nine".