647666021 has 2 divisors, whose sum is σ = 647666022. Its totient is φ = 647666020.

The previous prime is 647666011. The next prime is 647666023. The reversal of 647666021 is 120666746.

647666021 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 429069796 + 218596225 = 20714^2 + 14785^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-647666021 is a prime.

Together with 647666023, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (647666023) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 323833010 + 323833011.

It is an arithmetic number, because the mean of its divisors is an integer number (323833011).

Almost surely, 2^647666021 is an apocalyptic number.

It is an amenable number.

647666021 is a deficient number, since it is larger than the sum of its proper divisors (1).

647666021 is an equidigital number, since it uses as much as digits as its factorization.

647666021 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 72576, while the sum is 38.

The square root of 647666021 is about 25449.2833101445. The cubic root of 647666021 is about 865.2010496395.

The spelling of 647666021 in words is "six hundred forty-seven million, six hundred sixty-six thousand, twenty-one".