21999 has 4 divisors (see below), whose sum is σ = 29336.
Its totient is φ = 14664.
The previous prime is 21997. The next prime is 22003. The reversal of 21999 is 99912.
It can be divided in two parts, 2199 and 9, that multiplied together give a palindrome (19791).
It is a semiprime because it is the product of two primes.
It is not a de Polignac number, because 21999 - 21 = 21997 is a prime.
It is a D-number.
It is a Duffinian number.
21999 is a modest number, since divided by 999 gives 21 as remainder.
It is a plaindrome in base 11 and base 16.
It is a congruent number.
It is not an unprimeable number, because it can be changed into a prime (21991) by changing a digit.
It is a pernicious number, because its binary representation contains a prime number (11) of ones.
It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3664 + ... + 3669.
It is an arithmetic number, because the mean of its divisors is an integer number (7334).
221999 is an apocalyptic number.
21999 is a deficient number, since it is larger than the sum of its proper divisors (7337).
21999 is an equidigital number, since it uses as much as digits as its factorization.
21999 is an odious number, because the sum of its binary digits is odd.
The sum of its prime factors is 7336.
The product of its digits is 1458, while the sum is 30.
The square root of 21999 is about 148.3205987043.
The cubic root of 21999 is about 28.0199687487.
The spelling of 21999 in words is "twenty-one thousand, nine hundred ninety-nine".