1565616721 has 4 divisors (see below), whose sum is σ = 1573564212. Its totient is φ = 1557669232.

The previous prime is 1565616677. The next prime is 1565616733. The reversal of 1565616721 is 1276165651.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 1276165651 = 19553 ⋅65267.

It can be written as a sum of positive squares in 2 ways, for example, as 755645121 + 809971600 = 27489^2 + 28460^2 .

It is a cyclic number.

It is not a de Polignac number, because 1565616721 - 2⁹ = 1565616209 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1565616821) 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, 3973450 + ... + 3973843.

It is an arithmetic number, because the mean of its divisors is an integer number (393391053).

Almost surely, 2^1565616721 is an apocalyptic number.

It is an amenable number.

1565616721 is a deficient number, since it is larger than the sum of its proper divisors (7947491).

1565616721 is an equidigital number, since it uses as much as digits as its factorization.

1565616721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7947490.

The product of its digits is 75600, while the sum is 40.

The square root of 1565616721 is about 39567.8748608009. The cubic root of 1565616721 is about 1161.1682446287.

The spelling of 1565616721 in words is "one billion, five hundred sixty-five million, six hundred sixteen thousand, seven hundred twenty-one".