3991 has 4 divisors (see below), whose sum is σ = 4312. Its totient is φ = 3672.

The previous prime is 3989. The next prime is 4001. The reversal of 3991 is 1993.

3991 = T₁₈ + T₁₉ + ... + T₃₀.

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 3991 - 2¹ = 3989 is a prime.

It is an Ulam number.

It is a Duffinian number.

3991 is a lucky number.

It is a nialpdrome in base 16.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (3911) 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, 141 + ... + 166.

It is an arithmetic number, because the mean of its divisors is an integer number (1078).

2³⁹⁹¹ is an apocalyptic number.

3991 is a deficient number, since it is larger than the sum of its proper divisors (321).

3991 is a wasteful number, since it uses less digits than its factorization.

3991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 320.

The product of its digits is 243, while the sum is 22.

The square root of 3991 is about 63.1743618883. The cubic root of 3991 is about 15.8620960715.

Subtracting from 3991 its sum of digits (22), we obtain a square (3969 = 63²).

It can be divided in two parts, 399 and 1, that added together give a square (400 = 20²).

The spelling of 3991 in words is "three thousand, nine hundred ninety-one".