28820721 has 4 divisors (see below), whose sum is σ = 38427632. Its totient is φ = 19213812.

The previous prime is 28820719. The next prime is 28820723. The reversal of 28820721 is 12702882.

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.

It is an interprime number because it is at equal distance from previous prime (28820719) and next prime (28820723).

It is not a de Polignac number, because 28820721 - 2¹ = 28820719 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 (28820723) 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, 4803451 + ... + 4803456.

It is an arithmetic number, because the mean of its divisors is an integer number (9606908).

Almost surely, 2^28820721 is an apocalyptic number.

It is an amenable number.

28820721 is a deficient number, since it is larger than the sum of its proper divisors (9606911).

28820721 is an equidigital number, since it uses as much as digits as its factorization.

28820721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9606910.

The product of its (nonzero) digits is 3584, while the sum is 30.

The square root of 28820721 is about 5368.4933640641. The cubic root of 28820721 is about 306.5972678238.

The spelling of 28820721 in words is "twenty-eight million, eight hundred twenty thousand, seven hundred twenty-one".