1681991 has 2 divisors, whose sum is σ = 1681992. Its totient is φ = 1681990.

The previous prime is 1681973. The next prime is 1682017. The reversal of 1681991 is 1991861.

It is a weak prime.

It is an emirp because it is prime and its reverse (1991861) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 1681991 - 2¹⁰ = 1680967 is a prime.

It is a Chen prime.

It is a plaindrome in base 15.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (1681931) 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, 840995 + 840996.

It is an arithmetic number, because the mean of its divisors is an integer number (840996).

2^1681991 is an apocalyptic number.

1681991 is a deficient number, since it is larger than the sum of its proper divisors (1).

1681991 is an equidigital number, since it uses as much as digits as its factorization.

1681991 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 3888, while the sum is 35.

The square root of 1681991 is about 1296.9159571846. The cubic root of 1681991 is about 118.9253822166.

The spelling of 1681991 in words is "one million, six hundred eighty-one thousand, nine hundred ninety-one".