10991 has 4 divisors (see below), whose sum is σ = 11400. Its totient is φ = 10584.

The previous prime is 10987. The next prime is 10993. The reversal of 10991 is 19901.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 19901 = 7 ⋅2843.

It is a cyclic number.

It is not a de Polignac number, because 10991 - 2² = 10987 is a prime.

It is a Duffinian number.

It is a plaindrome in base 16.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (10993) 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, 161 + ... + 218.

It is an arithmetic number, because the mean of its divisors is an integer number (2850).

2¹⁰⁹⁹¹ is an apocalyptic number.

10991 is a deficient number, since it is larger than the sum of its proper divisors (409).

10991 is an equidigital number, since it uses as much as digits as its factorization.

10991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 408.

The product of its (nonzero) digits is 81, while the sum is 20.

The square root of 10991 is about 104.8379702207. The cubic root of 10991 is about 22.2337338505.

Adding to 10991 its sum of digits (20), we get a palindrome (11011).

It can be divided in two parts, 10 and 991, that added together give a palindrome (1001).

The spelling of 10991 in words is "ten thousand, nine hundred ninety-one".