661747721 has 2 divisors, whose sum is σ = 661747722. Its totient is φ = 661747720.

The previous prime is 661747711. The next prime is 661747747. The reversal of 661747721 is 127747166.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 603439225 + 58308496 = 24565^2 + 7636^2 .

It is a cyclic number.

It is not a de Polignac number, because 661747721 - 2¹⁴ = 661731337 is a prime.

It is not a weakly prime, because it can be changed into another prime (661747711) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 330873860 + 330873861.

It is an arithmetic number, because the mean of its divisors is an integer number (330873861).

Almost surely, 2^661747721 is an apocalyptic number.

It is an amenable number.

661747721 is a deficient number, since it is larger than the sum of its proper divisors (1).

661747721 is an equidigital number, since it uses as much as digits as its factorization.

661747721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 98784, while the sum is 41.

The square root of 661747721 is about 25724.4576424849. The cubic root of 661747721 is about 871.4266112534.

The spelling of 661747721 in words is "six hundred sixty-one million, seven hundred forty-seven thousand, seven hundred twenty-one".