1666747721 has 2 divisors, whose sum is σ = 1666747722. Its totient is φ = 1666747720.

The previous prime is 1666747673. The next prime is 1666747723. The reversal of 1666747721 is 1277476661.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 1665537721 + 1210000 = 40811^2 + 1100^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-1666747721 is a prime.

Together with 1666747723, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (1666747723) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 833373860 + 833373861.

It is an arithmetic number, because the mean of its divisors is an integer number (833373861).

Almost surely, 2^1666747721 is an apocalyptic number.

It is an amenable number.

1666747721 is a deficient number, since it is larger than the sum of its proper divisors (1).

1666747721 is an equidigital number, since it uses as much as digits as its factorization.

1666747721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 592704, while the sum is 47.

The square root of 1666747721 is about 40825.8217431076. The cubic root of 1666747721 is about 1185.6503212928.

The spelling of 1666747721 in words is "one billion, six hundred sixty-six million, seven hundred forty-seven thousand, seven hundred twenty-one".