16721665 has 4 divisors (see below), whose sum is σ = 20066004. Its totient is φ = 13377328.

The previous prime is 16721657. The next prime is 16721671. The reversal of 16721665 is 56612761.

It is a happy number.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 56612761 = 19 ⋅2979619.

It can be written as a sum of positive squares in 2 ways, for example, as 12159169 + 4562496 = 3487^2 + 2136^2 .

It is a cyclic number.

It is not a de Polignac number, because 16721665 - 2³ = 16721657 is a prime.

It is a super-2 number, since 2×16721665² = 559228160744450, which contains 22 as substring.

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 16721665.

It is an unprimeable number.

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, 1672162 + ... + 1672171.

It is an arithmetic number, because the mean of its divisors is an integer number (5016501).

Almost surely, 2^16721665 is an apocalyptic number.

It is an amenable number.

16721665 is a deficient number, since it is larger than the sum of its proper divisors (3344339).

16721665 is an equidigital number, since it uses as much as digits as its factorization.

16721665 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3344338.

The product of its digits is 15120, while the sum is 34.

The square root of 16721665 is about 4089.2132495139. The cubic root of 16721665 is about 255.7171405811.

It can be divided in two parts, 167216 and 65, that added together give a square (167281 = 409²).

The spelling of 16721665 in words is "sixteen million, seven hundred twenty-one thousand, six hundred sixty-five".