721562657681 has 4 divisors (see below), whose sum is σ = 726829538532. Its totient is φ = 716295776832.

The previous prime is 721562657657. The next prime is 721562657719. The reversal of 721562657681 is 186756265127.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 186756265127 = 523 ⋅357086549.

It can be written as a sum of positive squares in 2 ways, for example, as 285592979281 + 435969678400 = 534409^2 + 660280^2 .

It is a cyclic number.

It is not a de Polignac number, because 721562657681 - 2¹⁴ = 721562641297 is a prime.

It is a super-2 number, since 2×721562657681² (a number of 25 digits) contains 22 as substring.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (721562657611) 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, 2633440220 + ... + 2633440493.

It is an arithmetic number, because the mean of its divisors is an integer number (181707384633).

Almost surely, 2^721562657681 is an apocalyptic number.

It is an amenable number.

721562657681 is a deficient number, since it is larger than the sum of its proper divisors (5266880851).

721562657681 is a wasteful number, since it uses less digits than its factorization.

721562657681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 5266880850.

The product of its digits is 8467200, while the sum is 56.

The spelling of 721562657681 in words is "seven hundred twenty-one billion, five hundred sixty-two million, six hundred fifty-seven thousand, six hundred eighty-one".