891991 has 2 divisors, whose sum is σ = 891992. Its totient is φ = 891990.

The previous prime is 891983. The next prime is 891997. The reversal of 891991 is 199198.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 891991 - 2³ = 891983 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 891991.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (891997) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 445995 + 445996.

It is an arithmetic number, because the mean of its divisors is an integer number (445996).

2⁸⁹¹⁹⁹¹ is an apocalyptic number.

891991 is a deficient number, since it is larger than the sum of its proper divisors (1).

891991 is an equidigital number, since it uses as much as digits as its factorization.

891991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 5832, while the sum is 37.

The square root of 891991 is about 944.4527515975. The cubic root of 891991 is about 96.2616919512.

It can be divided in two parts, 89 and 1991, that added together give a triangular number (2080 = T₆₄).

The spelling of 891991 in words is "eight hundred ninety-one thousand, nine hundred ninety-one".