1991555 has 4 divisors (see below), whose sum is σ = 2389872. Its totient is φ = 1593240.

The previous prime is 1991551. The next prime is 1991573. The reversal of 1991555 is 5551991.

It is a happy number.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 5551991 = 157 ⋅35363.

It is not a de Polignac number, because 1991555 - 2² = 1991551 is a prime.

It is a Duffinian number.

It is a zygodrome in base 2.

It is not an unprimeable number, because it can be changed into a prime (1991551) 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, 199151 + ... + 199160.

It is an arithmetic number, because the mean of its divisors is an integer number (597468).

2^1991555 is an apocalyptic number.

1991555 is a deficient number, since it is larger than the sum of its proper divisors (398317).

1991555 is an equidigital number, since it uses as much as digits as its factorization.

1991555 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 398316.

The product of its digits is 10125, while the sum is 35.

The square root of 1991555 is about 1411.2246454764. The cubic root of 1991555 is about 125.8145209171.

The spelling of 1991555 in words is "one million, nine hundred ninety-one thousand, five hundred fifty-five".