685 has 4 divisors (see below), whose sum is σ = 828. Its totient is φ = 544.

The previous prime is 683. The next prime is 691. The reversal of 685 is 586.

685 = T₁₄ + T₁₅ + ... + T₁₈.

685 = 18² + 19².

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 586 = 2 ⋅293.

It can be written as a sum of positive squares in 2 ways, for example, as 324 + 361 = 18^2 + 19^2 .

It is a cyclic number.

It is not a de Polignac number, because 685 - 2¹ = 683 is a prime.

It is an Ulam number.

It is a Duffinian number.

685 is a lucky number.

It is a plaindrome in base 7, base 8, base 14 and base 16.

It is a nialpdrome in base 9.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (683) 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, 64 + ... + 73.

It is an arithmetic number, because the mean of its divisors is an integer number (207).

685 is the 19-th centered square number.

It is an amenable number.

685 is a deficient number, since it is larger than the sum of its proper divisors (143).

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

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

The sum of its prime factors is 142.

The product of its digits is 240, while the sum is 19.

The square root of 685 is about 26.1725046566. The cubic root of 685 is about 8.8151598195.

Subtracting from 685 its sum of digits (19), we obtain a palindrome (666).

Subtracting from 685 its reverse (586), we obtain a palindrome (99).

It can be divided in two parts, 6 and 85, that added together give a triangular number (91 = T₁₃).

The spelling of 685 in words is "six hundred eighty-five", and thus it is an aban number and an oban number.