21557 has 2 divisors, whose sum is σ = 21558.
Its totient is φ = 21556.
The previous prime is 21529. The next prime is 21559. The reversal of 21557 is 75512.
It is a strong prime.
It can be written as a sum of positive squares in only one way, i.e., 14161 + 7396 = 119^2 + 86^2
It is a cyclic number.
It is not a de Polignac number, because 21557 - 26 = 21493 is a prime.
Together with 21559, it forms a pair of twin primes.
It is a Chen prime.
It is a congruent number.
It is not a weakly prime, because it can be changed into another prime (21559) by changing a digit.
It is a pernicious number, because its binary representation contains a prime number (7) of ones.
It is a good prime.
It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10778 + 10779.
It is an arithmetic number, because the mean of its divisors is an integer number (10779).
221557 is an apocalyptic number.
It is an amenable number.
21557 is a deficient number, since it is larger than the sum of its proper divisors (1).
21557 is an equidigital number, since it uses as much as digits as its factorization.
21557 is an odious number, because the sum of its binary digits is odd.
The product of its digits is 350, while the sum is 20.
The square root of 21557 is about 146.8230227178.
The cubic root of 21557 is about 27.8310403249.
It can be divided in two parts, 215 and 57, that added together give a palindrome (272).
The spelling of 21557 in words is "twenty-one thousand, five hundred fifty-seven".