781991 has 6 divisors (see below), whose sum is σ = 909720. Its totient is φ = 670236.

The previous prime is 781987. The next prime is 781997. The reversal of 781991 is 199187.

It is not a de Polignac number, because 781991 - 2² = 781987 is a prime.

It is a super-2 number, since 2×781991² = 1223019848162, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (781997) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 7931 + ... + 8028.

It is an arithmetic number, because the mean of its divisors is an integer number (151620).

2⁷⁸¹⁹⁹¹ is an apocalyptic number.

781991 is a deficient number, since it is larger than the sum of its proper divisors (127729).

781991 is a wasteful number, since it uses less digits than its factorization.

781991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 15973 (or 15966 counting only the distinct ones).

The product of its digits is 4536, while the sum is 35.

The square root of 781991 is about 884.3025500359. The cubic root of 781991 is about 92.1298968497.

The spelling of 781991 in words is "seven hundred eighty-one thousand, nine hundred ninety-one".