16628421721 has 4 divisors (see below), whose sum is σ = 16633153252. Its totient is φ = 16623690192.

The previous prime is 16628421707. The next prime is 16628421743. The reversal of 16628421721 is 12712482661.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 12712482661 = 31907 ⋅398423.

It can be written as a sum of positive squares in 2 ways, for example, as 16317252121 + 311169600 = 127739^2 + 17640^2 .

It is a cyclic number.

It is not a de Polignac number, because 16628421721 - 2¹¹ = 16628419673 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (16628421781) 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, 2360490 + ... + 2367523.

It is an arithmetic number, because the mean of its divisors is an integer number (4158288313).

Almost surely, 2^16628421721 is an apocalyptic number.

It is an amenable number.

16628421721 is a deficient number, since it is larger than the sum of its proper divisors (4731531).

16628421721 is an equidigital number, since it uses as much as digits as its factorization.

16628421721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4731530.

The product of its digits is 64512, while the sum is 40.

The spelling of 16628421721 in words is "sixteen billion, six hundred twenty-eight million, four hundred twenty-one thousand, seven hundred twenty-one".