21764721 has 16 divisors (see below), whose sum is σ = 31785408. Its totient is φ = 13137600.

The previous prime is 21764711. The next prime is 21764723. The reversal of 21764721 is 12746712.

It is not a de Polignac number, because 21764721 - 2⁷ = 21764593 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21764723) 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 15 ways as a sum of consecutive naturals, for example, 7951 + ... + 10331.

It is an arithmetic number, because the mean of its divisors is an integer number (1986588).

Almost surely, 2^21764721 is an apocalyptic number.

It is an amenable number.

21764721 is a deficient number, since it is larger than the sum of its proper divisors (10020687).

21764721 is a wasteful number, since it uses less digits than its factorization.

21764721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2672.

The product of its digits is 4704, while the sum is 30.

The square root of 21764721 is about 4665.2675164453. The cubic root of 21764721 is about 279.2014705946.

Subtracting from 21764721 its reverse (12746712), we obtain a square (9018009 = 3003²).

The spelling of 21764721 in words is "twenty-one million, seven hundred sixty-four thousand, seven hundred twenty-one".