77991 has 4 divisors (see below), whose sum is σ = 103992. Its totient is φ = 51992.

The previous prime is 77983. The next prime is 77999. The reversal of 77991 is 19977.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 19977 = 3 ⋅6659.

It is an interprime number because it is at equal distance from previous prime (77983) and next prime (77999).

It is a cyclic number.

It is not a de Polignac number, because 77991 - 2³ = 77983 is a prime.

It is a D-number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (77999) 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, 12996 + ... + 13001.

It is an arithmetic number, because the mean of its divisors is an integer number (25998).

2⁷⁷⁹⁹¹ is an apocalyptic number.

77991 is a deficient number, since it is larger than the sum of its proper divisors (26001).

77991 is a wasteful number, since it uses less digits than its factorization.

77991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 26000.

The product of its digits is 3969, while the sum is 33.

The square root of 77991 is about 279.2686878259. The cubic root of 77991 is about 42.7249434235.

The spelling of 77991 in words is "seventy-seven thousand, nine hundred ninety-one".