21523 has 2 divisors, whose sum is σ = 21524. Its totient is φ = 21522.

The previous prime is 21521. The next prime is 21529. The reversal of 21523 is 32512.

It is a weak prime.

21523 is a truncatable prime.

It is a cyclic number.

It is not a de Polignac number, because 21523 - 2¹ = 21521 is a prime.

Together with 21521, it forms a pair of twin primes.

It is equal to p₂₄₁₆ and since 21523 and 2416 have the same sum of digits, it is a Honaker prime.

It is not a weakly prime, because it can be changed into another prime (21521) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10761 + 10762.

It is an arithmetic number, because the mean of its divisors is an integer number (10762).

2²¹⁵²³ is an apocalyptic number.

21523 is a deficient number, since it is larger than the sum of its proper divisors (1).

21523 is an equidigital number, since it uses as much as digits as its factorization.

21523 is an evil number, because the sum of its binary digits is even.

The product of its digits is 60, while the sum is 13.

The square root of 21523 is about 146.7071913711. The cubic root of 21523 is about 27.8164007911.

The spelling of 21523 in words is "twenty-one thousand, five hundred twenty-three".