21653 has 4 divisors (see below), whose sum is σ = 22080. Its totient is φ = 21228.

The previous prime is 21649. The next prime is 21661. The reversal of 21653 is 35612.

21653 is nontrivially palindromic in base 2 and base 8.

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 21653 - 2² = 21649 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21613) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 125 + ... + 242.

It is an arithmetic number, because the mean of its divisors is an integer number (5520).

2²¹⁶⁵³ is an apocalyptic number.

It is an amenable number.

21653 is a deficient number, since it is larger than the sum of its proper divisors (427).

21653 is an equidigital number, since it uses as much as digits as its factorization.

21653 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 426.

The product of its digits is 180, while the sum is 17.

The square root of 21653 is about 147.1495837575. The cubic root of 21653 is about 27.8722925642.

The spelling of 21653 in words is "twenty-one thousand, six hundred fifty-three".