21623 has 4 divisors (see below), whose sum is σ = 24720. Its totient is φ = 18528.

The previous prime is 21617. The next prime is 21647. The reversal of 21623 is 32612.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21623 - 2⁶ = 21559 is a prime.

It is a fibodiv number, since the Fibonacci-like sequence with seeds 21 and 623 contains 21623 itself.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21598 and 21607.

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 polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1538 + ... + 1551.

It is an arithmetic number, because the mean of its divisors is an integer number (6180).

2²¹⁶²³ is an apocalyptic number.

21623 is a deficient number, since it is larger than the sum of its proper divisors (3097).

21623 is an equidigital number, since it uses as much as digits as its factorization.

21623 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3096.

The product of its digits is 72, while the sum is 14.

The square root of 21623 is about 147.0476113373. The cubic root of 21623 is about 27.8594143603.

Subtracting from 21623 its sum of digits (14), we obtain a square (21609 = 147²).

The spelling of 21623 in words is "twenty-one thousand, six hundred twenty-three".