2166778721 has 2 divisors, whose sum is σ = 2166778722. Its totient is φ = 2166778720.

The previous prime is 2166778699. The next prime is 2166778739. The reversal of 2166778721 is 1278776612.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 1893207121 + 273571600 = 43511^2 + 16540^2 .

It is a cyclic number.

It is not a de Polignac number, because 2166778721 - 2¹⁸ = 2166516577 is a prime.

It is not a weakly prime, because it can be changed into another prime (2166778751) 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, 1083389360 + 1083389361.

It is an arithmetic number, because the mean of its divisors is an integer number (1083389361).

Almost surely, 2^2166778721 is an apocalyptic number.

It is an amenable number.

2166778721 is a deficient number, since it is larger than the sum of its proper divisors (1).

2166778721 is an equidigital number, since it uses as much as digits as its factorization.

2166778721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 395136, while the sum is 47.

The square root of 2166778721 is about 46548.6704536231. The cubic root of 2166778721 is about 1294.0116529731.

The spelling of 2166778721 in words is "two billion, one hundred sixty-six million, seven hundred seventy-eight thousand, seven hundred twenty-one".