665721557 has 2 divisors, whose sum is σ = 665721558. Its totient is φ = 665721556.

The previous prime is 665721541. The next prime is 665721607. The reversal of 665721557 is 755127566.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 638775076 + 26946481 = 25274^2 + 5191^2 .

It is a cyclic number.

It is not a de Polignac number, because 665721557 - 2⁴ = 665721541 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (665721517) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 332860778 + 332860779.

It is an arithmetic number, because the mean of its divisors is an integer number (332860779).

Almost surely, 2^665721557 is an apocalyptic number.

It is an amenable number.

665721557 is a deficient number, since it is larger than the sum of its proper divisors (1).

665721557 is an equidigital number, since it uses as much as digits as its factorization.

665721557 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 441000, while the sum is 44.

The square root of 665721557 is about 25801.5805136042. The cubic root of 665721557 is about 873.1674548340.

The spelling of 665721557 in words is "six hundred sixty-five million, seven hundred twenty-one thousand, five hundred fifty-seven".