21541 has 4 divisors (see below), whose sum is σ = 23212. Its totient is φ = 19872.

The previous prime is 21529. The next prime is 21557. The reversal of 21541 is 14512.

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

It can be written as a sum of positive squares in 2 ways, for example, as 4900 + 16641 = 70^2 + 129^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21541 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (13), and also a Moran number because the ratio is a prime number: 1657 = 21541 / (2 + 1 + 5 + 4 + 1).

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21521) 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, 816 + ... + 841.

It is an arithmetic number, because the mean of its divisors is an integer number (5803).

2²¹⁵⁴¹ is an apocalyptic number.

It is an amenable number.

21541 is a deficient number, since it is larger than the sum of its proper divisors (1671).

21541 is a wasteful number, since it uses less digits than its factorization.

21541 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1670.

The product of its digits is 40, while the sum is 13.

The square root of 21541 is about 146.7685252362. The cubic root of 21541 is about 27.8241530515.

Subtracting from 21541 its sum of digits (13), we obtain a triangular number (21528 = T₂₀₇).

It can be divided in two parts, 215 and 41, that added together give a 8-th power (256 = 2⁸).

The spelling of 21541 in words is "twenty-one thousand, five hundred forty-one".