691991 has 2 divisors, whose sum is σ = 691992. Its totient is φ = 691990.

The previous prime is 691979. The next prime is 691997. The reversal of 691991 is 199196.

It is a strong prime. It is also a bemirp because it and its reverse can be mirrored producing other two distinct primes, 166169 and 961661.

It is a cyclic number.

It is not a de Polignac number, because 691991 - 2¹⁴ = 675607 is a prime.

It is a Sophie Germain 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 (691997) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 345995 + 345996.

It is an arithmetic number, because the mean of its divisors is an integer number (345996).

2⁶⁹¹⁹⁹¹ is an apocalyptic number.

691991 is a deficient number, since it is larger than the sum of its proper divisors (1).

691991 is an equidigital number, since it uses as much as digits as its factorization.

691991 is an odious number, because the sum of its binary digits is odd.

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

The square root of 691991 is about 831.8599641767. The cubic root of 691991 is about 88.4504707592.

The spelling of 691991 in words is "six hundred ninety-one thousand, nine hundred ninety-one".