691 has 2 divisors, whose sum is σ = 692.
Its totient is φ = 690.
The previous prime is 683. The next prime is 701. The reversal of 691 is 196.
It is a weak prime.
It is a cyclic number.
It is not a de Polignac number, because 691 - 23 = 683 is a prime.
It is a plaindrome in base 11.
It is a nialpdrome in base 6 and base 15.
It is not a weakly prime, because it can be changed into another prime (601) by changing a digit.
It is a polite number, since it can be written as a sum of consecutive naturals, namely, 345 + 346.
It is an arithmetic number, because the mean of its divisors is an integer number (346).
691 is a deficient number, since it is larger than the sum of its proper divisors (1).
691 is an equidigital number, since it uses as much as digits as its factorization.
691 is an evil number, because the sum of its binary digits is even.
The product of its digits is 54, while the sum is 16.
The square root of 691 is about 26.2868788562.
The cubic root of 691 is about 8.8408227294.
Adding to 691 its sum of digits (16), we get a palindrome (707).
The spelling of 691 in words is "six hundred ninety-one", and thus it is an aban number.