21531 has 4 divisors (see below), whose sum is σ = 28712. Its totient is φ = 14352.

The previous prime is 21529. The next prime is 21557. The reversal of 21531 is 13512.

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

It is not a de Polignac number, because 21531 - 2¹ = 21529 is a prime.

It is a super-2 number, since 2×21531² = 927167922, which contains 22 as substring.

It is a D-number.

It is a Duffinian number.

21531 is a lucky number.

It is a plaindrome in base 14.

It is not an unprimeable number, because it can be changed into a prime (21521) 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, 3586 + ... + 3591.

It is an arithmetic number, because the mean of its divisors is an integer number (7178).

2²¹⁵³¹ is an apocalyptic number.

21531 is a deficient number, since it is larger than the sum of its proper divisors (7181).

21531 is an equidigital number, since it uses as much as digits as its factorization.

21531 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7180.

The product of its digits is 30, while the sum is 12.

The square root of 21531 is about 146.7344540318. The cubic root of 21531 is about 27.8198467735.

The spelling of 21531 in words is "twenty-one thousand, five hundred thirty-one".