6645721 has 2 divisors, whose sum is σ = 6645722. Its totient is φ = 6645720.

The previous prime is 6645713. The next prime is 6645739. The reversal of 6645721 is 1275466.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 3956121 + 2689600 = 1989^2 + 1640^2 .

It is a cyclic number.

It is not a de Polignac number, because 6645721 - 2³ = 6645713 is a prime.

It is not a weakly prime, because it can be changed into another prime (6648721) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 3322860 + 3322861.

It is an arithmetic number, because the mean of its divisors is an integer number (3322861).

Almost surely, 2^6645721 is an apocalyptic number.

It is an amenable number.

6645721 is a deficient number, since it is larger than the sum of its proper divisors (1).

6645721 is an equidigital number, since it uses as much as digits as its factorization.

6645721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 10080, while the sum is 31.

The square root of 6645721 is about 2577.9295956251. The cubic root of 6645721 is about 188.0098927192.

The spelling of 6645721 in words is "six million, six hundred forty-five thousand, seven hundred twenty-one".