1991857 has 4 divisors (see below), whose sum is σ = 2276416. Its totient is φ = 1707300.

The previous prime is 1991849. The next prime is 1991861. The reversal of 1991857 is 7581991.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 7581991 = 137 ⋅55343.

It is not a de Polignac number, because 1991857 - 2³ = 1991849 is a prime.

It is an Ulam number.

It is a Duffinian number.

1991857 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (1991837) 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 in 3 ways as a sum of consecutive naturals, for example, 142269 + ... + 142282.

It is an arithmetic number, because the mean of its divisors is an integer number (569104).

2^1991857 is an apocalyptic number.

It is an amenable number.

1991857 is a deficient number, since it is larger than the sum of its proper divisors (284559).

1991857 is an equidigital number, since it uses as much as digits as its factorization.

1991857 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 284558.

The product of its digits is 22680, while the sum is 40.

The square root of 1991857 is about 1411.3316406855. The cubic root of 1991857 is about 125.8208801130.

The spelling of 1991857 in words is "one million, nine hundred ninety-one thousand, eight hundred fifty-seven".