21542821 has 4 divisors (see below), whose sum is σ = 21896044. Its totient is φ = 21189600.

The previous prime is 21542813. The next prime is 21542831. The reversal of 21542821 is 12824512.

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 474721 + 21068100 = 689^2 + 4590^2 .

It is a cyclic number.

It is not a de Polignac number, because 21542821 - 2³ = 21542813 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 (21542831) 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 3 ways as a sum of consecutive naturals, for example, 176520 + ... + 176641.

It is an arithmetic number, because the mean of its divisors is an integer number (5474011).

Almost surely, 2^21542821 is an apocalyptic number.

It is an amenable number.

21542821 is a deficient number, since it is larger than the sum of its proper divisors (353223).

21542821 is an equidigital number, since it uses as much as digits as its factorization.

21542821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 353222.

The product of its digits is 1280, while the sum is 25.

The square root of 21542821 is about 4641.4244580732. The cubic root of 21542821 is about 278.2493708121.

The spelling of 21542821 in words is "twenty-one million, five hundred forty-two thousand, eight hundred twenty-one".