646985821 has 2 divisors, whose sum is σ = 646985822. Its totient is φ = 646985820.

The previous prime is 646985819. The next prime is 646985873. The reversal of 646985821 is 128589646.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 519612025 + 127373796 = 22795^2 + 11286^2 .

It is a cyclic number.

It is not a de Polignac number, because 646985821 - 2¹ = 646985819 is a prime.

Together with 646985819, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (646989821) 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, 323492910 + 323492911.

It is an arithmetic number, because the mean of its divisors is an integer number (323492911).

Almost surely, 2^646985821 is an apocalyptic number.

It is an amenable number.

646985821 is a deficient number, since it is larger than the sum of its proper divisors (1).

646985821 is an equidigital number, since it uses as much as digits as its factorization.

646985821 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 829440, while the sum is 49.

The square root of 646985821 is about 25435.9159654218. The cubic root of 646985821 is about 864.8980560804.

The spelling of 646985821 in words is "six hundred forty-six million, nine hundred eighty-five thousand, eight hundred twenty-one".