861991 has 8 divisors (see below), whose sum is σ = 944384. Its totient is φ = 781920.

The previous prime is 861979. The next prime is 861997. The reversal of 861991 is 199168.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 861991 - 2¹³ = 853799 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (861997) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 250 + ... + 1336.

It is an arithmetic number, because the mean of its divisors is an integer number (118048).

2⁸⁶¹⁹⁹¹ is an apocalyptic number.

861991 is a deficient number, since it is larger than the sum of its proper divisors (82393).

861991 is a wasteful number, since it uses less digits than its factorization.

861991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1161.

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

The square root of 861991 is about 928.4347042200. The cubic root of 861991 is about 95.1701843287.

The spelling of 861991 in words is "eight hundred sixty-one thousand, nine hundred ninety-one".