21942821 has 4 divisors (see below), whose sum is σ = 22699500. Its totient is φ = 21186144.

The previous prime is 21942799. The next prime is 21942827. The reversal of 21942821 is 12824912.

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 8473921 + 13468900 = 2911^2 + 3670^2 .

It is a cyclic number.

It is not a de Polignac number, because 21942821 - 2⁶ = 21942757 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (29), and also a Moran number because the ratio is a prime number: 756649 = 21942821 / (2 + 1 + 9 + 4 + 2 + 8 + 2 + 1).

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21942827) 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, 378296 + ... + 378353.

It is an arithmetic number, because the mean of its divisors is an integer number (5674875).

Almost surely, 2^21942821 is an apocalyptic number.

It is an amenable number.

21942821 is a deficient number, since it is larger than the sum of its proper divisors (756679).

21942821 is an equidigital number, since it uses as much as digits as its factorization.

21942821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 756678.

The product of its digits is 2304, while the sum is 29.

The square root of 21942821 is about 4684.3164922964. The cubic root of 21942821 is about 279.9609681987.

The spelling of 21942821 in words is "twenty-one million, nine hundred forty-two thousand, eight hundred twenty-one".