21721521 has 8 divisors (see below), whose sum is σ = 28993024. Its totient is φ = 14465520.

The previous prime is 21721519. The next prime is 21721549. The reversal of 21721521 is 12512712.

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

It is not a de Polignac number, because 21721521 - 2¹ = 21721519 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21721621) 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, 70 + ... + 6591.

It is an arithmetic number, because the mean of its divisors is an integer number (3624128).

Almost surely, 2^21721521 is an apocalyptic number.

It is an amenable number.

21721521 is a deficient number, since it is larger than the sum of its proper divisors (7271503).

21721521 is a wasteful number, since it uses less digits than its factorization.

21721521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7751.

The product of its digits is 280, while the sum is 21.

The square root of 21721521 is about 4660.6352571297. The cubic root of 21721521 is about 279.0166226398.

The spelling of 21721521 in words is "twenty-one million, seven hundred twenty-one thousand, five hundred twenty-one".