209821 has 2 divisors, whose sum is σ = 209822. Its totient is φ = 209820.

The previous prime is 209819. The next prime is 209837. The reversal of 209821 is 128902.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 164025 + 45796 = 405^2 + 214^2 .

It is a cyclic number.

It is not a de Polignac number, because 209821 - 2¹ = 209819 is a prime.

Together with 209819, it forms a pair of twin primes.

It is a Chen prime.

It is a plaindrome in base 16.

It is a junction number, because it is equal to n+sod(n) for n = 209792 and 209801.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (209801) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 104910 + 104911.

It is an arithmetic number, because the mean of its divisors is an integer number (104911).

2²⁰⁹⁸²¹ is an apocalyptic number.

It is an amenable number.

209821 is a deficient number, since it is larger than the sum of its proper divisors (1).

209821 is an equidigital number, since it uses as much as digits as its factorization.

209821 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 288, while the sum is 22.

The square root of 209821 is about 458.0622228475. The cubic root of 209821 is about 59.4223264408.

The spelling of 209821 in words is "two hundred nine thousand, eight hundred twenty-one".