687721 has 2 divisors, whose sum is σ = 687722. Its totient is φ = 687720.

The previous prime is 687707. The next prime is 687737. The reversal of 687721 is 127786.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 632025 + 55696 = 795^2 + 236^2 .

It is a cyclic number.

It is not a de Polignac number, because 687721 - 2⁷ = 687593 is a prime.

It is not a weakly prime, because it can be changed into another prime (687121) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 343860 + 343861.

It is an arithmetic number, because the mean of its divisors is an integer number (343861).

2⁶⁸⁷⁷²¹ is an apocalyptic number.

It is an amenable number.

687721 is a deficient number, since it is larger than the sum of its proper divisors (1).

687721 is an equidigital number, since it uses as much as digits as its factorization.

687721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 4704, while the sum is 31.

The square root of 687721 is about 829.2894548950. The cubic root of 687721 is about 88.2681644243.

The spelling of 687721 in words is "six hundred eighty-seven thousand, seven hundred twenty-one".