21991913 has 2 divisors, whose sum is σ = 21991914. Its totient is φ = 21991912.

The previous prime is 21991909. The next prime is 21991919. The reversal of 21991913 is 31919912.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 18939904 + 3052009 = 4352^2 + 1747^2 .

It is a cyclic number.

It is not a de Polignac number, because 21991913 - 2² = 21991909 is a prime.

It is not a weakly prime, because it can be changed into another prime (21991919) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10995956 + 10995957.

It is an arithmetic number, because the mean of its divisors is an integer number (10995957).

Almost surely, 2^21991913 is an apocalyptic number.

It is an amenable number.

21991913 is a deficient number, since it is larger than the sum of its proper divisors (1).

21991913 is an equidigital number, since it uses as much as digits as its factorization.

21991913 is an evil number, because the sum of its binary digits is even.

The product of its digits is 4374, while the sum is 35.

The square root of 21991913 is about 4689.5536034894. The cubic root of 21991913 is about 280.1695953850.

The spelling of 21991913 in words is "twenty-one million, nine hundred ninety-one thousand, nine hundred thirteen".