688693721 has 4 divisors (see below), whose sum is σ = 699983844. Its totient is φ = 677403600.

The previous prime is 688693699. The next prime is 688693723. The reversal of 688693721 is 127396886.

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 17106496 + 671587225 = 4136^2 + 25915^2 .

It is a cyclic number.

It is not a de Polignac number, because 688693721 - 2¹⁸ = 688431577 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 688693721.

It is not an unprimeable number, because it can be changed into a prime (688693723) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 5644970 + ... + 5645091.

It is an arithmetic number, because the mean of its divisors is an integer number (174995961).

Almost surely, 2^688693721 is an apocalyptic number.

688693721 is a gapful number since it is divisible by the number (61) formed by its first and last digit.

It is an amenable number.

688693721 is a deficient number, since it is larger than the sum of its proper divisors (11290123).

688693721 is a wasteful number, since it uses less digits than its factorization.

688693721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 11290122.

The product of its digits is 870912, while the sum is 50.

The square root of 688693721 is about 26242.9746980025. The cubic root of 688693721 is about 883.0976067645.

The spelling of 688693721 in words is "six hundred eighty-eight million, six hundred ninety-three thousand, seven hundred twenty-one".