991681 has 4 divisors (see below), whose sum is σ = 996352. Its totient is φ = 987012.

The previous prime is 991663. The next prime is 991693. The reversal of 991681 is 186199.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 186199 = 13 ⋅14323.

It is a cyclic number.

It is not a de Polignac number, because 991681 - 2¹⁷ = 860609 is a prime.

It is a super-2 number, since 2×991681² = 1966862411522, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (991621) 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, 2001 + ... + 2446.

It is an arithmetic number, because the mean of its divisors is an integer number (249088).

2⁹⁹¹⁶⁸¹ is an apocalyptic number.

It is an amenable number.

991681 is a deficient number, since it is larger than the sum of its proper divisors (4671).

991681 is a wasteful number, since it uses less digits than its factorization.

991681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4670.

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

The square root of 991681 is about 995.8318131090. The cubic root of 991681 is about 99.7219274734.

The spelling of 991681 in words is "nine hundred ninety-one thousand, six hundred eighty-one".