684397921 has 3 divisors (see below), whose sum is σ = 684424083. Its totient is φ = 684371760.

The previous prime is 684397919. The next prime is 684398009. The reversal of 684397921 is 129793486.

The square root of 684397921 is 26161.

It is a perfect power (a square), and thus also a powerful number.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 129793486 = 2 ⋅64896743.

It can be written as a sum of positive squares in only one way, i.e., 170015521 + 514382400 = 13039^2 + 22680^2 .

It is not a de Polignac number, because 684397921 - 2¹ = 684397919 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (684397421) 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 2 ways as a sum of consecutive naturals, for example, 13081 + ... + 39241.

It is an arithmetic number, because the mean of its divisors is an integer number (228141361).

Almost surely, 2^684397921 is an apocalyptic number.

684397921 is the 26161-st square number.

684397921 is the 13081-st centered octagonal number.

It is an amenable number.

684397921 is a deficient number, since it is larger than the sum of its proper divisors (26162).

684397921 is an frugal number, since it uses more digits than its factorization.

684397921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 52322 (or 26161 counting only the distinct ones).

The product of its digits is 653184, while the sum is 49.

The cubic root of 684397921 is about 881.2576375040.

Multiplying 684397921 by its sum of digits (49), we get a square (33535498129 = 183127²).

The spelling of 684397921 in words is "six hundred eighty-four million, three hundred ninety-seven thousand, nine hundred twenty-one".