721681 has 4 divisors (see below), whose sum is σ = 723492. Its totient is φ = 719872.

The previous prime is 721663. The next prime is 721687. The reversal of 721681 is 186127.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 186127 = 373 ⋅499.

It can be written as a sum of positive squares in 2 ways, for example, as 342225 + 379456 = 585^2 + 616^2 .

It is a cyclic number.

It is not a de Polignac number, because 721681 - 2⁹ = 721169 is a prime.

It is a super-2 number, since 2×721681² = 1041646931522, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (721687) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 16 + ... + 1201.

It is an arithmetic number, because the mean of its divisors is an integer number (180873).

2⁷²¹⁶⁸¹ is an apocalyptic number.

It is an amenable number.

721681 is a deficient number, since it is larger than the sum of its proper divisors (1811).

721681 is a wasteful number, since it uses less digits than its factorization.

721681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1810.

The product of its digits is 672, while the sum is 25.

The square root of 721681 is about 849.5180986889. The cubic root of 721681 is about 89.6977929524.

The spelling of 721681 in words is "seven hundred twenty-one thousand, six hundred eighty-one".