665561 has 4 divisors (see below), whose sum is σ = 716772. Its totient is φ = 614352.

The previous prime is 665557. The next prime is 665563. The reversal of 665561 is 165566.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 430336 + 235225 = 656^2 + 485^2 .

It is a cyclic number.

It is not a de Polignac number, because 665561 - 2² = 665557 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (665563) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 25586 + ... + 25611.

It is an arithmetic number, because the mean of its divisors is an integer number (179193).

2⁶⁶⁵⁵⁶¹ is an apocalyptic number.

It is an amenable number.

665561 is a deficient number, since it is larger than the sum of its proper divisors (51211).

665561 is a wasteful number, since it uses less digits than its factorization.

665561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 51210.

The product of its digits is 5400, while the sum is 29.

The square root of 665561 is about 815.8192201707. The cubic root of 665561 is about 87.3097253102.

The spelling of 665561 in words is "six hundred sixty-five thousand, five hundred sixty-one".