665561261 has 2 divisors, whose sum is σ = 665561262. Its totient is φ = 665561260.

The previous prime is 665561257. The next prime is 665561287. The reversal of 665561261 is 162165566.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 617522500 + 48038761 = 24850^2 + 6931^2 .

It is a cyclic number.

It is not a de Polignac number, because 665561261 - 2² = 665561257 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 (665561251) 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, 332780630 + 332780631.

It is an arithmetic number, because the mean of its divisors is an integer number (332780631).

Almost surely, 2^665561261 is an apocalyptic number.

It is an amenable number.

665561261 is a deficient number, since it is larger than the sum of its proper divisors (1).

665561261 is an equidigital number, since it uses as much as digits as its factorization.

665561261 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 64800, while the sum is 38.

The square root of 665561261 is about 25798.4740052585. The cubic root of 665561261 is about 873.0973672305.

The spelling of 665561261 in words is "six hundred sixty-five million, five hundred sixty-one thousand, two hundred sixty-one".