21864821 has 6 divisors (see below), whose sum is σ = 24033366. Its totient is φ = 19877000.

The previous prime is 21864797. The next prime is 21864853. The reversal of 21864821 is 12846812.

It is a happy number.

It can be written as a sum of positive squares in only one way, i.e., 12780625 + 9084196 = 3575^2 + 3014^2 .

It is not a de Polignac number, because 21864821 - 2⁶ = 21864757 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21864121) 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 5 ways as a sum of consecutive naturals, for example, 90230 + ... + 90471.

It is an arithmetic number, because the mean of its divisors is an integer number (4005561).

Almost surely, 2^21864821 is an apocalyptic number.

It is an amenable number.

21864821 is a deficient number, since it is larger than the sum of its proper divisors (2168545).

21864821 is a wasteful number, since it uses less digits than its factorization.

21864821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 180723 (or 180712 counting only the distinct ones).

The product of its digits is 6144, while the sum is 32.

The square root of 21864821 is about 4675.9834259758. The cubic root of 21864821 is about 279.6288492291.

Subtracting from 21864821 its reverse (12846812), we obtain a square (9018009 = 3003²).

It can be divided in two parts, 2186 and 4821, that added together give a palindrome (7007).

The spelling of 21864821 in words is "twenty-one million, eight hundred sixty-four thousand, eight hundred twenty-one".