66991 has 4 divisors (see below), whose sum is σ = 69184. Its totient is φ = 64800.

The previous prime is 66977. The next prime is 67003. The reversal of 66991 is 19966.

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

It is a 5-Lehmer number, since φ(66991) divides (66991-1)⁵.

It is a cyclic number.

It is not a de Polignac number, because 66991 - 2⁵ = 66959 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (31), and also a Moran number because the ratio is a prime number: 2161 = 66991 / (6 + 6 + 9 + 9 + 1).

It is a deceptive number, since it divides R₆₆₉₉₀.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (66931) 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, 1050 + ... + 1111.

It is an arithmetic number, because the mean of its divisors is an integer number (17296).

2⁶⁶⁹⁹¹ is an apocalyptic number.

66991 is a deficient number, since it is larger than the sum of its proper divisors (2193).

66991 is a wasteful number, since it uses less digits than its factorization.

66991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2192.

The product of its digits is 2916, while the sum is 31.

The square root of 66991 is about 258.8261965103. The cubic root of 66991 is about 40.6136623194.

The spelling of 66991 in words is "sixty-six thousand, nine hundred ninety-one".