21240824821 has 4 divisors (see below), whose sum is σ = 21692757312. Its totient is φ = 20788892332.

The previous prime is 21240824747. The next prime is 21240824849. The reversal of 21240824821 is 12842804212.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21240824821 - 2⁹ = 21240824309 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21240824861) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 225966175 + ... + 225966268.

It is an arithmetic number, because the mean of its divisors is an integer number (5423189328).

Almost surely, 2^21240824821 is an apocalyptic number.

It is an amenable number.

21240824821 is a deficient number, since it is larger than the sum of its proper divisors (451932491).

21240824821 is an equidigital number, since it uses as much as digits as its factorization.

21240824821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 451932490.

The product of its (nonzero) digits is 16384, while the sum is 34.

The spelling of 21240824821 in words is "twenty-one billion, two hundred forty million, eight hundred twenty-four thousand, eight hundred twenty-one".