1645818721 has 2 divisors, whose sum is σ = 1645818722. Its totient is φ = 1645818720.

The previous prime is 1645818709. The next prime is 1645818731. The reversal of 1645818721 is 1278185461.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 1008761121 + 637057600 = 31761^2 + 25240^2 .

It is a cyclic number.

It is not a de Polignac number, because 1645818721 - 2⁵ = 1645818689 is a prime.

It is not a weakly prime, because it can be changed into another prime (1645818731) 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, 822909360 + 822909361.

It is an arithmetic number, because the mean of its divisors is an integer number (822909361).

Almost surely, 2^1645818721 is an apocalyptic number.

It is an amenable number.

1645818721 is a deficient number, since it is larger than the sum of its proper divisors (1).

1645818721 is an equidigital number, since it uses as much as digits as its factorization.

1645818721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 107520, while the sum is 43.

The square root of 1645818721 is about 40568.6913888038. The cubic root of 1645818721 is about 1180.6667497314. Note that the first 3 decimals are identical.

The spelling of 1645818721 in words is "one billion, six hundred forty-five million, eight hundred eighteen thousand, seven hundred twenty-one".