21867821 has 2 divisors, whose sum is σ = 21867822. Its totient is φ = 21867820.

The previous prime is 21867773. The next prime is 21867823. The reversal of 21867821 is 12876812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 14523721 + 7344100 = 3811^2 + 2710^2 .

It is a cyclic number.

It is not a de Polignac number, because 21867821 - 2⁶ = 21867757 is a prime.

Together with 21867823, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21867823) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10933910 + 10933911.

It is an arithmetic number, because the mean of its divisors is an integer number (10933911).

Almost surely, 2^21867821 is an apocalyptic number.

It is an amenable number.

21867821 is a deficient number, since it is larger than the sum of its proper divisors (1).

21867821 is an equidigital number, since it uses as much as digits as its factorization.

21867821 is an evil number, because the sum of its binary digits is even.

The product of its digits is 10752, while the sum is 35.

The square root of 21867821 is about 4676.3042031074. The cubic root of 21867821 is about 279.6416376284.

The spelling of 21867821 in words is "twenty-one million, eight hundred sixty-seven thousand, eight hundred twenty-one".