665891 has 4 divisors (see below), whose sum is σ = 674400. Its totient is φ = 657384.

The previous prime is 665857. The next prime is 665897. The reversal of 665891 is 198566.

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 665891 - 2¹⁴ = 649507 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 (665897) 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, 4136 + ... + 4293.

It is an arithmetic number, because the mean of its divisors is an integer number (168600).

2⁶⁶⁵⁸⁹¹ is an apocalyptic number.

665891 is a deficient number, since it is larger than the sum of its proper divisors (8509).

665891 is an equidigital number, since it uses as much as digits as its factorization.

665891 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 8508.

The product of its digits is 12960, while the sum is 35.

The square root of 665891 is about 816.0214457966. The cubic root of 665891 is about 87.3241529628.

Subtracting from 665891 its sum of digits (35), we obtain a square (665856 = 816²).

The spelling of 665891 in words is "six hundred sixty-five thousand, eight hundred ninety-one".