209816821 has 2 divisors, whose sum is σ = 209816822. Its totient is φ = 209816820.

The previous prime is 209816813. The next prime is 209816837. The reversal of 209816821 is 128618902.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 115455025 + 94361796 = 10745^2 + 9714^2 .

It is a cyclic number.

It is not a de Polignac number, because 209816821 - 2³ = 209816813 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (209816801) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 104908410 + 104908411.

It is an arithmetic number, because the mean of its divisors is an integer number (104908411).

Almost surely, 2^209816821 is an apocalyptic number.

It is an amenable number.

209816821 is a deficient number, since it is larger than the sum of its proper divisors (1).

209816821 is an equidigital number, since it uses as much as digits as its factorization.

209816821 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 13824, while the sum is 37.

The square root of 209816821 is about 14485.0550913692. The cubic root of 209816821 is about 594.2193193377.

The spelling of 209816821 in words is "two hundred nine million, eight hundred sixteen thousand, eight hundred twenty-one".