665271721 has 4 divisors (see below), whose sum is σ = 669255552. Its totient is φ = 661287892.

The previous prime is 665271713. The next prime is 665271769. The reversal of 665271721 is 127172566.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 665271721 - 2³ = 665271713 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (665271521) 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, 1991665 + ... + 1991998.

It is an arithmetic number, because the mean of its divisors is an integer number (167313888).

Almost surely, 2^665271721 is an apocalyptic number.

It is an amenable number.

665271721 is a deficient number, since it is larger than the sum of its proper divisors (3983831).

665271721 is a wasteful number, since it uses less digits than its factorization.

665271721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3983830.

The product of its digits is 35280, while the sum is 37.

The square root of 665271721 is about 25792.8618226051. The cubic root of 665271721 is about 872.9707406305.

The spelling of 665271721 in words is "six hundred sixty-five million, two hundred seventy-one thousand, seven hundred twenty-one".