15999 has 4 divisors (see below), whose sum is σ = 21336. Its totient is φ = 10664.

The previous prime is 15991. The next prime is 16001. The reversal of 15999 is 99951.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 99951 = 3 ⋅33317.

It is a cyclic number.

It is not a de Polignac number, because 15999 - 2³ = 15991 is a prime.

It is a D-number.

15999 is a modest number, since divided by 999 gives 15 as remainder.

It is a plaindrome in base 10.

It is a zygodrome in base 2.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (15991) 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, 2664 + ... + 2669.

It is an arithmetic number, because the mean of its divisors is an integer number (5334).

2¹⁵⁹⁹⁹ is an apocalyptic number.

15999 is a deficient number, since it is larger than the sum of its proper divisors (5337).

15999 is an equidigital number, since it uses as much as digits as its factorization.

15999 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5336.

The product of its digits is 3645, while the sum is 33.

The square root of 15999 is about 126.4871534979. The cubic root of 15999 is about 25.1978960199.

The spelling of 15999 in words is "fifteen thousand, nine hundred ninety-nine".