21998 has 8 divisors (see below), whose sum is σ = 34992. Its totient is φ = 10336.

The previous prime is 21997. The next prime is 22003. The reversal of 21998 is 89912.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a Curzon number.

It is a plaindrome in base 11 and base 16.

It is a zygodrome in base 16.

It is a self number, because there is not a number n which added to its sum of digits gives 21998.

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 polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 290 + ... + 357.

It is an arithmetic number, because the mean of its divisors is an integer number (4374).

2²¹⁹⁹⁸ is an apocalyptic number.

21998 is a deficient number, since it is larger than the sum of its proper divisors (12994).

21998 is a wasteful number, since it uses less digits than its factorization.

21998 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 666.

The product of its digits is 1296, while the sum is 29.

The square root of 21998 is about 148.3172275901. The cubic root of 21998 is about 28.0195441780.

Subtracting from 21998 its product of digits (1296), we obtain a palindrome (20702).

The spelling of 21998 in words is "twenty-one thousand, nine hundred ninety-eight".