721561321 has 4 divisors (see below), whose sum is σ = 777066052. Its totient is φ = 666056592.

The previous prime is 721561319. The next prime is 721561403. The reversal of 721561321 is 123165127.

It is a happy number.

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 282072025 + 439489296 = 16795^2 + 20964^2 .

It is a cyclic number.

It is not a de Polignac number, because 721561321 - 2¹ = 721561319 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 721561321.

It is not an unprimeable number, because it can be changed into a prime (721561021) 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 3 ways as a sum of consecutive naturals, for example, 27752346 + ... + 27752371.

It is an arithmetic number, because the mean of its divisors is an integer number (194266513).

Almost surely, 2^721561321 is an apocalyptic number.

It is an amenable number.

721561321 is a deficient number, since it is larger than the sum of its proper divisors (55504731).

721561321 is a wasteful number, since it uses less digits than its factorization.

721561321 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 55504730.

The product of its digits is 2520, while the sum is 28.

The square root of 721561321 is about 26861.8934738413. The cubic root of 721561321 is about 896.9283437393.

The spelling of 721561321 in words is "seven hundred twenty-one million, five hundred sixty-one thousand, three hundred twenty-one".