21991 has 2 divisors, whose sum is σ = 21992. Its totient is φ = 21990.

The previous prime is 21977. The next prime is 21997. The reversal of 21991 is 19912.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21991 - 2⁷ = 21863 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 (21997) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10995 + 10996.

It is an arithmetic number, because the mean of its divisors is an integer number (10996).

2²¹⁹⁹¹ is an apocalyptic number.

21991 is a deficient number, since it is larger than the sum of its proper divisors (1).

21991 is an equidigital number, since it uses as much as digits as its factorization.

21991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 162, while the sum is 22.

The square root of 21991 is about 148.2936276446. The cubic root of 21991 is about 28.0165718227.

The spelling of 21991 in words is "twenty-one thousand, nine hundred ninety-one".