1981991 has 4 divisors (see below), whose sum is σ = 2162184. Its totient is φ = 1801800.

The previous prime is 1981963. The next prime is 1981997. The reversal of 1981991 is 1991891.

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: 1991891 = 11 ⋅181081.

It is a de Polignac number, because none of the positive numbers 2^k-1981991 is a prime.

It is a super-4 number, since 4×1981991⁴ (a number of 26 digits) contains 4444 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1981997) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 90080 + ... + 90101.

It is an arithmetic number, because the mean of its divisors is an integer number (540546).

2^1981991 is an apocalyptic number.

1981991 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

1981991 is a deficient number, since it is larger than the sum of its proper divisors (180193).

1981991 is a wasteful number, since it uses less digits than its factorization.

1981991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 180192.

The product of its digits is 5832, while the sum is 38.

The square root of 1981991 is about 1407.8320212298. The cubic root of 1981991 is about 125.6127989107.

It can be divided in two parts, 1981 and 991, that multiplied together give a triangular number (1963171 = T₁₉₈₁).

The spelling of 1981991 in words is "one million, nine hundred eighty-one thousand, nine hundred ninety-one".