21993 has 4 divisors (see below), whose sum is σ = 29328. Its totient is φ = 14660.

The previous prime is 21991. The next prime is 21997. The reversal of 21993 is 39912.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21993 - 2¹ = 21991 is a prime.

It is a D-number.

It is a Curzon number.

21993 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (21991) 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, 3663 + ... + 3668.

It is an arithmetic number, because the mean of its divisors is an integer number (7332).

2²¹⁹⁹³ is an apocalyptic number.

It is an amenable number.

21993 is a deficient number, since it is larger than the sum of its proper divisors (7335).

21993 is an equidigital number, since it uses as much as digits as its factorization.

21993 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7334.

The product of its digits is 486, while the sum is 24.

The square root of 21993 is about 148.3003708694. The cubic root of 21993 is about 28.0174211314.

The spelling of 21993 in words is "twenty-one thousand, nine hundred ninety-three".