16556821 has 4 divisors (see below), whose sum is σ = 17090944. Its totient is φ = 16022700.

The previous prime is 16556807. The next prime is 16556851. The reversal of 16556821 is 12865561.

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: 12865561 = 233 ⋅55217.

It is a cyclic number.

It is not a de Polignac number, because 16556821 - 2⁵ = 16556789 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (16556851) by changing a digit.

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, 267015 + ... + 267076.

It is an arithmetic number, because the mean of its divisors is an integer number (4272736).

Almost surely, 2^16556821 is an apocalyptic number.

It is an amenable number.

16556821 is a deficient number, since it is larger than the sum of its proper divisors (534123).

16556821 is an equidigital number, since it uses as much as digits as its factorization.

16556821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 534122.

The product of its digits is 14400, while the sum is 34.

The square root of 16556821 is about 4069.0073728122. The cubic root of 16556821 is about 254.8740682815.

The spelling of 16556821 in words is "sixteen million, five hundred fifty-six thousand, eight hundred twenty-one".