21547 has 4 divisors (see below), whose sum is σ = 22320. Its totient is φ = 20776.

The previous prime is 21529. The next prime is 21557. The reversal of 21547 is 74512.

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 21547 - 2⁷ = 21419 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21517) 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, 343 + ... + 400.

It is an arithmetic number, because the mean of its divisors is an integer number (5580).

2²¹⁵⁴⁷ is an apocalyptic number.

21547 is a deficient number, since it is larger than the sum of its proper divisors (773).

21547 is an equidigital number, since it uses as much as digits as its factorization.

21547 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 772.

The product of its digits is 280, while the sum is 19.

The square root of 21547 is about 146.7889641628. The cubic root of 21547 is about 27.8267361785.

Subtracting from 21547 its sum of digits (19), we obtain a triangular number (21528 = T₂₀₇).

It can be divided in two parts, 215 and 47, that added together give a palindrome (262).

The spelling of 21547 in words is "twenty-one thousand, five hundred forty-seven".