688404721 has 8 divisors (see below), whose sum is σ = 710923304. Its totient is φ = 666092160.

The previous prime is 688404677. The next prime is 688404737. The reversal of 688404721 is 127404886.

It can be written as a sum of positive squares in 4 ways, for example, as 171479025 + 516925696 = 13095^2 + 22736^2 .

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 688404721 - 2⁷ = 688404593 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (688404781) 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, 44700 + ... + 58093.

It is an arithmetic number, because the mean of its divisors is an integer number (88865413).

Almost surely, 2^688404721 is an apocalyptic number.

It is an amenable number.

688404721 is a deficient number, since it is larger than the sum of its proper divisors (22518583).

688404721 is a wasteful number, since it uses less digits than its factorization.

688404721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 103011.

The product of its (nonzero) digits is 86016, while the sum is 40.

The square root of 688404721 is about 26237.4678846873. The cubic root of 688404721 is about 882.9740632560.

The spelling of 688404721 in words is "six hundred eighty-eight million, four hundred four thousand, seven hundred twenty-one".