668729821 has 8 divisors (see below), whose sum is σ = 694009080. Its totient is φ = 643598592.

The previous prime is 668729813. The next prime is 668729843. The reversal of 668729821 is 128927866.

It can be written as a sum of positive squares in 4 ways, for example, as 53144100 + 615585721 = 7290^2 + 24811^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 668729821 - 2³ = 668729813 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (668729801) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 27760 + ... + 45913.

It is an arithmetic number, because the mean of its divisors is an integer number (86751135).

Almost surely, 2^668729821 is an apocalyptic number.

It is an amenable number.

668729821 is a deficient number, since it is larger than the sum of its proper divisors (25279259).

668729821 is a wasteful number, since it uses less digits than its factorization.

668729821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 74015.

The product of its digits is 580608, while the sum is 49.

The square root of 668729821 is about 25859.8109235160. The cubic root of 668729821 is about 874.4807023701.

The spelling of 668729821 in words is "six hundred sixty-eight million, seven hundred twenty-nine thousand, eight hundred twenty-one".