16557721 has 4 divisors (see below), whose sum is σ = 17429200. Its totient is φ = 15686244.

The previous prime is 16557719. The next prime is 16557791. The reversal of 16557721 is 12775561.

It is a happy number.

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, and also an emirpimes, since its reverse is a distinct semiprime: 12775561 = 157 ⋅81373.

It is a cyclic number.

It is not a de Polignac number, because 16557721 - 2¹ = 16557719 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (16557791) 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, 435711 + ... + 435748.

It is an arithmetic number, because the mean of its divisors is an integer number (4357300).

Almost surely, 2^16557721 is an apocalyptic number.

It is an amenable number.

16557721 is a deficient number, since it is larger than the sum of its proper divisors (871479).

16557721 is an equidigital number, since it uses as much as digits as its factorization.

16557721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 871478.

The product of its digits is 14700, while the sum is 34.

The square root of 16557721 is about 4069.1179633921. The cubic root of 16557721 is about 254.8786863682.

The spelling of 16557721 in words is "sixteen million, five hundred fifty-seven thousand, seven hundred twenty-one".