1991881 has 2 divisors, whose sum is σ = 1991882. Its totient is φ = 1991880.

The previous prime is 1991879. The next prime is 1991893. The reversal of 1991881 is 1881991.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 1798281 + 193600 = 1341^2 + 440^2 .

It is a cyclic number.

It is not a de Polignac number, because 1991881 - 2¹ = 1991879 is a prime.

It is a super-2 number, since 2×1991881² = 7935179836322, which contains 22 as substring.

Together with 1991879, it forms a pair of twin primes.

It is a Chen prime.

1991881 is a lucky number.

It is a self number, because there is not a number n which added to its sum of digits gives 1991881.

It is not a weakly prime, because it can be changed into another prime (1991861) 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, 995940 + 995941.

It is an arithmetic number, because the mean of its divisors is an integer number (995941).

2^1991881 is an apocalyptic number.

It is an amenable number.

1991881 is a deficient number, since it is larger than the sum of its proper divisors (1).

1991881 is an equidigital number, since it uses as much as digits as its factorization.

1991881 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 5184, while the sum is 37.

The square root of 1991881 is about 1411.3401432681. The cubic root of 1991881 is about 125.8213854519.

It can be divided in two parts, 199 and 1881, that added together give a triangular number (2080 = T₆₄).

The spelling of 1991881 in words is "one million, nine hundred ninety-one thousand, eight hundred eighty-one".