16561721 has 4 divisors (see below), whose sum is σ = 18067344. Its totient is φ = 15056100.

The previous prime is 16561717. The next prime is 16561763. The reversal of 16561721 is 12716561.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 16561721 - 2² = 16561717 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (16560721) 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, 752795 + ... + 752816.

It is an arithmetic number, because the mean of its divisors is an integer number (4516836).

Almost surely, 2^16561721 is an apocalyptic number.

16561721 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

It is an amenable number.

16561721 is a deficient number, since it is larger than the sum of its proper divisors (1505623).

16561721 is a wasteful number, since it uses less digits than its factorization.

16561721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1505622.

The product of its digits is 2520, while the sum is 29.

The square root of 16561721 is about 4069.6094407203. The cubic root of 16561721 is about 254.8992091736.

The spelling of 16561721 in words is "sixteen million, five hundred sixty-one thousand, seven hundred twenty-one".