21540821 has 4 divisors (see below), whose sum is σ = 21630444. Its totient is φ = 21451200.

The previous prime is 21540817. The next prime is 21540833. The reversal of 21540821 is 12804512.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 7214596 + 14326225 = 2686^2 + 3785^2 .

It is a cyclic number.

It is not a de Polignac number, because 21540821 - 2² = 21540817 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 (21540221) 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, 44450 + ... + 44931.

It is an arithmetic number, because the mean of its divisors is an integer number (5407611).

Almost surely, 2^21540821 is an apocalyptic number.

It is an amenable number.

21540821 is a deficient number, since it is larger than the sum of its proper divisors (89623).

21540821 is an equidigital number, since it uses as much as digits as its factorization.

21540821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 89622.

The product of its (nonzero) digits is 640, while the sum is 23.

The square root of 21540821 is about 4641.2090019735. The cubic root of 21540821 is about 278.2407598080.

The spelling of 21540821 in words is "twenty-one million, five hundred forty thousand, eight hundred twenty-one".