21991439 has 2 divisors, whose sum is σ = 21991440. Its totient is φ = 21991438.

The previous prime is 21991433. The next prime is 21991451. The reversal of 21991439 is 93419912.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21991439 - 2²⁰ = 20942863 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21991433) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10995719 + 10995720.

It is an arithmetic number, because the mean of its divisors is an integer number (10995720).

Almost surely, 2^21991439 is an apocalyptic number.

21991439 is a deficient number, since it is larger than the sum of its proper divisors (1).

21991439 is an equidigital number, since it uses as much as digits as its factorization.

21991439 is an evil number, because the sum of its binary digits is even.

The product of its digits is 17496, while the sum is 38.

The square root of 21991439 is about 4689.5030653578. The cubic root of 21991439 is about 280.1675825035.

The spelling of 21991439 in words is "twenty-one million, nine hundred ninety-one thousand, four hundred thirty-nine".