21491 has 2 divisors, whose sum is σ = 21492.
Its totient is φ = 21490.
The previous prime is 21487. The next prime is 21493. The reversal of 21491 is 19412.
It can be divided in two parts, 21 and 491, that added together give a 9-th power (512 = 29).
It is a happy number.
It is a strong prime.
It is a cyclic number.
It is not a de Polignac number, because 21491 - 22 = 21487 is a prime.
Together with 21493, it forms a pair of twin primes.
It is a Chen prime.
It is not a weakly prime, because it can be changed into another prime (21493) by changing a digit.
It is a good prime.
It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10745 + 10746.
It is an arithmetic number, because the mean of its divisors is an integer number (10746).
221491 is an apocalyptic number.
21491 is a deficient number, since it is larger than the sum of its proper divisors (1).
21491 is an equidigital number, since it uses as much as digits as its factorization.
21491 is an evil number, because the sum of its binary digits is even.
The product of its digits is 72, while the sum is 17.
The square root of 21491 is about 146.5980900285.
The cubic root of 21491 is about 27.8026083159.
The spelling of 21491 in words is "twenty-one thousand, four hundred ninety-one".