669721 has 4 divisors (see below), whose sum is σ = 721252. Its totient is φ = 618192.

The previous prime is 669707. The next prime is 669733. The reversal of 669721 is 127966.

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 404496 + 265225 = 636^2 + 515^2 .

It is a cyclic number.

It is not a de Polignac number, because 669721 - 2⁵ = 669689 is a prime.

It is a Duffinian number.

669721 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (669701) 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, 25746 + ... + 25771.

It is an arithmetic number, because the mean of its divisors is an integer number (180313).

2⁶⁶⁹⁷²¹ is an apocalyptic number.

It is an amenable number.

669721 is a deficient number, since it is larger than the sum of its proper divisors (51531).

669721 is a wasteful number, since it uses less digits than its factorization.

669721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 51530.

The product of its digits is 4536, while the sum is 31.

The square root of 669721 is about 818.3648330665. The cubic root of 669721 is about 87.4912535475.

The spelling of 669721 in words is "six hundred sixty-nine thousand, seven hundred twenty-one".