21724921 has 9 divisors (see below), whose sum is σ = 22382661. Its totient is φ = 21086364.

The previous prime is 21724907. The next prime is 21724931. The reversal of 21724921 is 12942712.

The square root of 21724921 is 4661.

It is a perfect power (a square), and thus also a powerful number.

It is not a de Polignac number, because 21724921 - 2¹¹ = 21722873 is a prime.

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 21724921.

It is not an unprimeable number, because it can be changed into a prime (21724931) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 8 ways as a sum of consecutive naturals, for example, 274960 + ... + 275038.

Almost surely, 2^21724921 is an apocalyptic number.

21724921 is the 4661-st square number.

21724921 is the 2331-st centered octagonal number.

It is an amenable number.

21724921 is a deficient number, since it is larger than the sum of its proper divisors (657740).

21724921 is an frugal number, since it uses more digits than its factorization.

21724921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 276 (or 138 counting only the distinct ones).

The product of its digits is 2016, while the sum is 28.

The cubic root of 21724921 is about 279.0311797393.

The spelling of 21724921 in words is "twenty-one million, seven hundred twenty-four thousand, nine hundred twenty-one".