21991919 has 2 divisors, whose sum is σ = 21991920. Its totient is φ = 21991918.

The previous prime is 21991913. The next prime is 21991939. The reversal of 21991919 is 91919912.

It is a happy number.

It is a weak prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21991919 is a prime.

It is a super-2 number, since 2×21991919² = 967289002605122, which contains 22 as substring.

It is a Sophie Germain prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21991913) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10995959 + 10995960.

It is an arithmetic number, because the mean of its divisors is an integer number (10995960).

Almost surely, 2^21991919 is an apocalyptic number.

21991919 is a deficient number, since it is larger than the sum of its proper divisors (1).

21991919 is an equidigital number, since it uses as much as digits as its factorization.

21991919 is an evil number, because the sum of its binary digits is even.

The product of its digits is 13122, while the sum is 41.

The square root of 21991919 is about 4689.5542432090. The cubic root of 21991919 is about 280.1696208643.

The spelling of 21991919 in words is "twenty-one million, nine hundred ninety-one thousand, nine hundred nineteen".