21729921 has 8 divisors (see below), whose sum is σ = 29647200. Its totient is φ = 14149632.

The previous prime is 21729919. The next prime is 21729931. The reversal of 21729921 is 12992712.

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

It is not a de Polignac number, because 21729921 - 2¹ = 21729919 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21729931) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 84096 + ... + 84353.

It is an arithmetic number, because the mean of its divisors is an integer number (3705900).

Almost surely, 2^21729921 is an apocalyptic number.

It is an amenable number.

21729921 is a deficient number, since it is larger than the sum of its proper divisors (7917279).

21729921 is a wasteful number, since it uses less digits than its factorization.

21729921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 168495.

The product of its digits is 4536, while the sum is 33.

The square root of 21729921 is about 4661.5363347291. The cubic root of 21729921 is about 279.0525844797.

The spelling of 21729921 in words is "twenty-one million, seven hundred twenty-nine thousand, nine hundred twenty-one".