19991 has 2 divisors, whose sum is σ = 19992.
Its totient is φ = 19990.
The previous prime is 19979. The next prime is 19993.
19991 is nontrivially palindromic in base 10.
It is a strong prime.
It is a palprime.
It is a cyclic number.
It is not a de Polignac number, because 19991 - 26 = 19927 is a prime.
It is a Sophie Germain prime.
Together with 19993, 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 (19993) by changing a digit.
It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9995 + 9996.
It is an arithmetic number, because the mean of its divisors is an integer number (9996).
219991 is an apocalyptic number.
19991 is a deficient number, since it is larger than the sum of its proper divisors (1).
19991 is an equidigital number, since it uses as much as digits as its factorization.
19991 is an evil number, because the sum of its binary digits is even.
The product of its digits is 729, while the sum is 29.
The square root of 19991 is about 141.3895328516.
The cubic root of 19991 is about 27.1401039286.
The spelling of 19991 in words is "nineteen thousand, nine hundred ninety-one".