1991781 has 36 divisors (see below), whose sum is σ = 3319680. Its totient is φ = 1148400.

The previous prime is 1991779. The next prime is 1991797. The reversal of 1991781 is 1871991.

It is not a de Polignac number, because 1991781 - 2¹ = 1991779 is a prime.

It is a super-2 number, since 2×1991781² = 7934383103922, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1991761) 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 35 ways as a sum of consecutive naturals, for example, 33730 + ... + 33788.

2^1991781 is an apocalyptic number.

1991781 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

It is an amenable number.

1991781 is a deficient number, since it is larger than the sum of its proper divisors (1327899).

1991781 is a wasteful number, since it uses less digits than its factorization.

1991781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 118 (or 104 counting only the distinct ones).

The product of its digits is 4536, while the sum is 36.

The square root of 1991781 is about 1411.3047155026. The cubic root of 1991781 is about 125.8192798461.

It can be divided in two parts, 1991 and 781, that added together give a palindrome (2772).

The spelling of 1991781 in words is "one million, nine hundred ninety-one thousand, seven hundred eighty-one".