665821661 has 2 divisors, whose sum is σ = 665821662. Its totient is φ = 665821660.

The previous prime is 665821573. The next prime is 665821699. The reversal of 665821661 is 166128566.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 428076100 + 237745561 = 20690^2 + 15419^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-665821661 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (665821361) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 332910830 + 332910831.

It is an arithmetic number, because the mean of its divisors is an integer number (332910831).

Almost surely, 2^665821661 is an apocalyptic number.

It is an amenable number.

665821661 is a deficient number, since it is larger than the sum of its proper divisors (1).

665821661 is an equidigital number, since it uses as much as digits as its factorization.

665821661 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 103680, while the sum is 41.

The square root of 665821661 is about 25803.5203218476. The cubic root of 665821661 is about 873.2112184632.

The spelling of 665821661 in words is "six hundred sixty-five million, eight hundred twenty-one thousand, six hundred sixty-one".