216821921 has 2 divisors, whose sum is σ = 216821922. Its totient is φ = 216821920.

The previous prime is 216821809. The next prime is 216821933. The reversal of 216821921 is 129128612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 116100625 + 100721296 = 10775^2 + 10036^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-216821921 is a prime.

It is a super-2 number, since 2×216821921² = 94023490852260482, which contains 22 as substring.

It is a Sophie Germain prime.

It is a Chen prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (216821971) 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, 108410960 + 108410961.

It is an arithmetic number, because the mean of its divisors is an integer number (108410961).

Almost surely, 2^216821921 is an apocalyptic number.

It is an amenable number.

216821921 is a deficient number, since it is larger than the sum of its proper divisors (1).

216821921 is an equidigital number, since it uses as much as digits as its factorization.

216821921 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 3456, while the sum is 32.

The square root of 216821921 is about 14724.8742269671. The cubic root of 216821921 is about 600.7600747005.

The spelling of 216821921 in words is "two hundred sixteen million, eight hundred twenty-one thousand, nine hundred twenty-one".