21521 has 2 divisors, whose sum is σ = 21522.
Its totient is φ = 21520.
The previous prime is 21517. The next prime is 21523. The reversal of 21521 is 12512.
It is a strong prime.
It can be written as a sum of positive squares in only one way, i.e., 18496 + 3025 = 136^2 + 55^2
It is a cyclic number.
It is not a de Polignac number, because 21521 - 22 = 21517 is a prime.
Together with 21523, it forms a pair of twin primes.
It is a Chen prime.
It is a nialpdrome in base 16.
It is not a weakly prime, because it can be changed into another prime (21523) by changing a digit.
It is a pernicious number, because its binary representation contains a prime number (5) of ones.
It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10760 + 10761.
It is an arithmetic number, because the mean of its divisors is an integer number (10761).
221521 is an apocalyptic number.
It is an amenable number.
21521 is a deficient number, since it is larger than the sum of its proper divisors (1).
21521 is an equidigital number, since it uses as much as digits as its factorization.
21521 is an odious number, because the sum of its binary digits is odd.
The product of its digits is 20, while the sum is 11.
The square root of 21521 is about 146.7003749143.
The cubic root of 21521 is about 27.8155391621.
Subtracting from 21521 its reverse (12512), we obtain a palindrome (9009).
The spelling of 21521 in words is "twenty-one thousand, five hundred twenty-one".