665691601 has 3 divisors (see below), whose sum is σ = 665717403. Its totient is φ = 665665800.

The previous prime is 665691589. The next prime is 665691617. The reversal of 665691601 is 106196566.

The square root of 665691601 is 25801.

It is a perfect power (a square), and thus also a powerful number.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in only one way, i.e., 345997201 + 319694400 = 18601^2 + 17880^2 .

It is not a de Polignac number, because 665691601 - 2⁵ = 665691569 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (665691671) 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 in 2 ways as a sum of consecutive naturals, for example, 12901 + ... + 38701.

It is an arithmetic number, because the mean of its divisors is an integer number (221905801).

Almost surely, 2^665691601 is an apocalyptic number.

665691601 is the 25801-st square number.

665691601 is the 12901-st centered octagonal number.

It is an amenable number.

665691601 is a deficient number, since it is larger than the sum of its proper divisors (25802).

665691601 is an frugal number, since it uses more digits than its factorization.

665691601 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 51602 (or 25801 counting only the distinct ones).

The product of its (nonzero) digits is 58320, while the sum is 40.

The cubic root of 665691601 is about 873.1543577685.

The spelling of 665691601 in words is "six hundred sixty-five million, six hundred ninety-one thousand, six hundred one".