1691 is nontrivially palindromic in base 12.
It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 1961 = 37 ⋅53.
It is a cyclic number.
1691 is a strobogrammatic number because it is the same when read upside-down.
It is a Duffinian number.
1691 is an undulating number in base 12.
It is a plaindrome in base 6, base 9, base 14, base 15 and base 16.
1691 is an equidigital number, since it uses as much as digits as its factorization.
1691 is an odious number, because the sum of its binary digits is odd.
The sum of its prime factors is 108.
The square root of 1691 is about 41.1217703899. The cubic root of 1691 is about 11.9137331742.
The spelling of 1691 in words is "one thousand, six hundred ninety-one".