665640121 has 4 divisors (see below), whose sum is σ = 666660132. Its totient is φ = 664620112.

The previous prime is 665640097. The next prime is 665640133. The reversal of 665640121 is 121046566.

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 154654096 + 510986025 = 12436^2 + 22605^2 .

It is a cyclic number.

It is not a de Polignac number, because 665640121 - 2⁵ = 665640089 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (665640181) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 509026 + ... + 510331.

It is an arithmetic number, because the mean of its divisors is an integer number (166665033).

Almost surely, 2^665640121 is an apocalyptic number.

It is an amenable number.

665640121 is a deficient number, since it is larger than the sum of its proper divisors (1020011).

665640121 is a wasteful number, since it uses less digits than its factorization.

665640121 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1020010.

The product of its (nonzero) digits is 8640, while the sum is 31.

The square root of 665640121 is about 25800.0023449611. The cubic root of 665640121 is about 873.1318492750.

Adding to 665640121 its reverse (121046566), we get a palindrome (786686687).

The spelling of 665640121 in words is "six hundred sixty-five million, six hundred forty thousand, one hundred twenty-one".