681991 has 4 divisors (see below), whose sum is σ = 685152. Its totient is φ = 678832.

The previous prime is 681983. The next prime is 681997. The reversal of 681991 is 199186.

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

It is a cyclic number.

It is not a de Polignac number, because 681991 - 2³ = 681983 is a prime.

It is a super-2 number, since 2×681991² = 930223448162, which contains 22 as substring.

It is a Duffinian number.

It is a nialpdrome in base 15.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (681997) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1231 + ... + 1696.

It is an arithmetic number, because the mean of its divisors is an integer number (171288).

2⁶⁸¹⁹⁹¹ is an apocalyptic number.

681991 is a deficient number, since it is larger than the sum of its proper divisors (3161).

681991 is a wasteful number, since it uses less digits than its factorization.

681991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3160.

The product of its digits is 3888, while the sum is 34.

The square root of 681991 is about 825.8274638204. The cubic root of 681991 is about 88.0223342072.

The spelling of 681991 in words is "six hundred eighty-one thousand, nine hundred ninety-one".