1781991 has 24 divisors (see below), whose sum is σ = 2873520. Its totient is φ = 1057536.

The previous prime is 1781981. The next prime is 1782043. The reversal of 1781991 is 1991871.

It is not a de Polignac number, because 1781991 - 2⁷ = 1781863 is a prime.

1781991 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1781921) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 2601 + ... + 3213.

It is an arithmetic number, because the mean of its divisors is an integer number (119730).

2^1781991 is an apocalyptic number.

1781991 is a deficient number, since it is larger than the sum of its proper divisors (1091529).

1781991 is a wasteful number, since it uses less digits than its factorization.

1781991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 655 (or 652 counting only the distinct ones).

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

The square root of 1781991 is about 1334.9123566737. The cubic root of 1781991 is about 121.2369965153.

It can be divided in two parts, 1781 and 991, that added together give a palindrome (2772).

The spelling of 1781991 in words is "one million, seven hundred eighty-one thousand, nine hundred ninety-one".