6991 has 2 divisors, whose sum is σ = 6992.
Its totient is φ = 6990.
The previous prime is 6983. The next prime is 6997. The reversal of 6991 is 1996.
6991 is digitally balanced in base 3, because in such base it contains all the possibile digits an equal number of times.
It is a strong prime.
It is also a bemirp because it and its reverse can be mirrored producing other two distinct primes, 1669 and 9661.
It is a cyclic number.
It is not a de Polignac number, because 6991 - 23 = 6983 is a prime.
It is a nialpdrome in base 6 and base 15.
It is a congruent number.
It is not a weakly prime, because it can be changed into another prime (6997) by changing a digit.
It is a polite number, since it can be written as a sum of consecutive naturals, namely, 3495 + 3496.
It is an arithmetic number, because the mean of its divisors is an integer number (3496).
26991 is an apocalyptic number.
6991 is a deficient number, since it is larger than the sum of its proper divisors (1).
6991 is an equidigital number, since it uses as much as digits as its factorization.
6991 is an odious number, because the sum of its binary digits is odd.
The product of its digits is 486, while the sum is 25.
The square root of 6991 is about 83.6122000667.
The cubic root of 6991 is about 19.1211100352.
The spelling of 6991 in words is "six thousand, nine hundred ninety-one".