391981 has 4 divisors (see below), whose sum is σ = 395964. Its totient is φ = 388000.

The previous prime is 391967. The next prime is 391987. The reversal of 391981 is 189193.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 324900 + 67081 = 570^2 + 259^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-391981 is a prime.

It is a super-2 number, since 2×391981² = 307298208722, which contains 22 as substring.

It is an Ulam number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (391987) 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, 1840 + ... + 2041.

It is an arithmetic number, because the mean of its divisors is an integer number (98991).

2³⁹¹⁹⁸¹ is an apocalyptic number.

It is an amenable number.

391981 is a deficient number, since it is larger than the sum of its proper divisors (3983).

391981 is a wasteful number, since it uses less digits than its factorization.

391981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3982.

The product of its digits is 1944, while the sum is 31.

The square root of 391981 is about 626.0838601977. The cubic root of 391981 is about 73.1849317526.

The spelling of 391981 in words is "three hundred ninety-one thousand, nine hundred eighty-one".