16991 has 4 divisors (see below), whose sum is σ = 18312. Its totient is φ = 15672.

The previous prime is 16987. The next prime is 16993. The reversal of 16991 is 19961.

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 16991 - 2² = 16987 is a prime.

It is a Duffinian number.

It is a plaindrome in base 12.

It is a zygodrome in base 12.

It is a self number, because there is not a number n which added to its sum of digits gives 16991.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 16991.

It is not an unprimeable number, because it can be changed into a prime (16993) 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, 641 + ... + 666.

It is an arithmetic number, because the mean of its divisors is an integer number (4578).

2¹⁶⁹⁹¹ is an apocalyptic number.

16991 is a deficient number, since it is larger than the sum of its proper divisors (1321).

16991 is a wasteful number, since it uses less digits than its factorization.

16991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1320.

The product of its digits is 486, while the sum is 26.

The square root of 16991 is about 130.3495301104. The cubic root of 16991 is about 25.7082775499.

The spelling of 16991 in words is "sixteen thousand, nine hundred ninety-one".