686655721 has 4 divisors (see below), whose sum is σ = 727047252. Its totient is φ = 646264192.

The previous prime is 686655719. The next prime is 686655773. The reversal of 686655721 is 127556686.

686655721 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

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 302064400 + 384591321 = 17380^2 + 19611^2 .

It is a cyclic number.

It is not a de Polignac number, because 686655721 - 2¹ = 686655719 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (686655791) 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, 20195740 + ... + 20195773.

It is an arithmetic number, because the mean of its divisors is an integer number (181761813).

Almost surely, 2^686655721 is an apocalyptic number.

It is an amenable number.

686655721 is a deficient number, since it is larger than the sum of its proper divisors (40391531).

686655721 is a wasteful number, since it uses less digits than its factorization.

686655721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 40391530.

The product of its digits is 604800, while the sum is 46.

The square root of 686655721 is about 26204.1164895900. The cubic root of 686655721 is about 882.2256511169.

The spelling of 686655721 in words is "six hundred eighty-six million, six hundred fifty-five thousand, seven hundred twenty-one".