856721561 has 2 divisors, whose sum is σ = 856721562. Its totient is φ = 856721560.

The previous prime is 856721539. The next prime is 856721581. The reversal of 856721561 is 165127658.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 821911561 + 34810000 = 28669^2 + 5900^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-856721561 is a prime.

It is a Chen prime.

It is equal to p_43908728 and since 856721561 and 43908728 have the same sum of digits, it is a Honaker prime.

It is not a weakly prime, because it can be changed into another prime (856721531) 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, 428360780 + 428360781.

It is an arithmetic number, because the mean of its divisors is an integer number (428360781).

Almost surely, 2^856721561 is an apocalyptic number.

It is an amenable number.

856721561 is a deficient number, since it is larger than the sum of its proper divisors (1).

856721561 is an equidigital number, since it uses as much as digits as its factorization.

856721561 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 100800, while the sum is 41.

The square root of 856721561 is about 29269.8063027414. The cubic root of 856721561 is about 949.7585946171.

The spelling of 856721561 in words is "eight hundred fifty-six million, seven hundred twenty-one thousand, five hundred sixty-one".