21663 has 12 divisors (see below), whose sum is σ = 32760. Its totient is φ = 13776.

The previous prime is 21661. The next prime is 21673. The reversal of 21663 is 36612.

It is a happy number.

21663 is a `hidden beast` number, since 2 + 1 + 663 = 666.

It is not a de Polignac number, because 21663 - 2¹ = 21661 is a prime.

21663 is a lucky number.

It is a nialpdrome in base 15.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21661) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 220 + ... + 302.

It is an arithmetic number, because the mean of its divisors is an integer number (2730).

2²¹⁶⁶³ is an apocalyptic number.

21663 is a deficient number, since it is larger than the sum of its proper divisors (11097).

21663 is a wasteful number, since it uses less digits than its factorization.

21663 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 118 (or 115 counting only the distinct ones).

The product of its digits is 216, while the sum is 18.

The square root of 21663 is about 147.1835588644. The cubic root of 21663 is about 27.8765826553.

The spelling of 21663 in words is "twenty-one thousand, six hundred sixty-three".